feat: allow users to specify a custom font (closes #176)

Adds support for loading a custom font from either a remote URL or a
local file path, and applying it to the terminal and input bar.

- Rust: adds `custom_font_path` and `custom_font_family` fields to
  `HikariConfig` with `#[serde(default)]` for backwards compatibility
- TypeScript: extends `HikariConfig` interface and `defaultConfig`;
  exports `applyCustomFont()` which injects an `@import` for CSS
  stylesheet URLs, a `@font-face` rule for direct font file URLs, or a
  base64 data URL `@font-face` for local files via Tauri `readFile`;
  adds `setCustomFont()` to the config store
- Terminal.svelte and InputBar.svelte now use
  `--terminal-font-family` CSS variable (falls back to `monospace`)
- ConfigSidebar.svelte: new "Custom Font" section with URL/path input,
  family name input, Apply + Reset buttons, and inline status feedback
- `+page.svelte`: applies saved font on startup alongside theme/size
- 14 new tests for `applyCustomFont` (all code paths) + 2 for
  `setCustomFont`

src-tauri/src/config.rs

@@ -144,6 +144,13 @@ pub struct HikariConfig {
 
     #[serde(default)]
     pub show_thinking_blocks: bool,
+
+    // Custom font settings
+    #[serde(default)]
+    pub custom_font_path: Option<String>,
+
+    #[serde(default)]
+    pub custom_font_family: Option<String>,
 }
 
 impl Default for HikariConfig {
@@ -183,6 +190,8 @@ impl Default for HikariConfig {
             background_image_path: None,
             background_image_opacity: 0.3,
             show_thinking_blocks: false,
+            custom_font_path: None,
+            custom_font_family: None,
         }
     }
 }
@@ -298,6 +307,8 @@ mod tests {
         assert!(!config.disable_1m_context);
         assert!(config.trusted_workspaces.is_empty());
         assert!(!config.show_thinking_blocks);
+        assert!(config.custom_font_path.is_none());
+        assert!(config.custom_font_family.is_none());
     }
 
     #[test]
@@ -337,6 +348,8 @@ mod tests {
             background_image_path: Some("/home/naomi/bg.png".to_string()),
             background_image_opacity: 0.25,
             show_thinking_blocks: true,
+            custom_font_path: Some("/home/naomi/.fonts/MyFont.ttf".to_string()),
+            custom_font_family: Some("MyFont".to_string()),
         };
 
         let json = serde_json::to_string(&config).unwrap();

src/lib/components/ConfigSidebar.svelte

@@ -5,6 +5,7 @@
     type Theme,
     type CustomThemeColors,
     applyFontSize,
+    applyCustomFont,
     applyCustomThemeColors,
     MIN_FONT_SIZE,
     MAX_FONT_SIZE,
@@ -60,9 +61,14 @@
     trusted_workspaces: [],
     background_image_path: null,
     background_image_opacity: 0.3,
+    custom_font_path: null,
+    custom_font_family: null,
   });
 
   let showCustomThemeEditor = $state(false);
+  let customFontPathInput = $state("");
+  let customFontFamilyInput = $state("");
+  let customFontStatus: string | null = $state(null);
 
   interface AuthStatus {
     is_logged_in: boolean;
@@ -88,6 +94,8 @@
 
   configStore.config.subscribe((c) => {
     config = { ...c };
+    customFontPathInput = c.custom_font_path ?? "";
+    customFontFamilyInput = c.custom_font_family ?? "";
   });
 
   configStore.isSidebarOpen.subscribe((open) => {
@@ -937,6 +945,68 @@
         </p>
       </div>
 
+      <!-- Custom Font -->
+      <div class="mb-4">
+        <span class="block text-sm text-[var(--text-secondary)] mb-2">Custom Font</span>
+        <div class="flex flex-col gap-2">
+          <input
+            type="text"
+            bind:value={customFontPathInput}
+            placeholder="URL or local file path (e.g. /path/to/font.ttf)"
+            class="w-full px-3 py-2 text-sm rounded-lg border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-primary)] placeholder-gray-500 focus:outline-none focus:border-[var(--accent-primary)]"
+          />
+          <input
+            type="text"
+            bind:value={customFontFamilyInput}
+            placeholder="Font family name (e.g. FiraCode)"
+            class="w-full px-3 py-2 text-sm rounded-lg border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-primary)] placeholder-gray-500 focus:outline-none focus:border-[var(--accent-primary)]"
+          />
+          <div class="flex gap-2">
+            <button
+              onclick={async () => {
+                customFontStatus = null;
+                try {
+                  await configStore.setCustomFont(
+                    customFontPathInput || null,
+                    customFontFamilyInput || null
+                  );
+                  customFontStatus = "Font applied!";
+                } catch (e) {
+                  customFontStatus = `Error: ${e instanceof Error ? e.message : String(e)}`;
+                }
+              }}
+              class="flex-1 px-3 py-2 text-sm rounded-lg border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-secondary)] hover:border-[var(--accent-primary)] transition-colors"
+            >
+              Apply
+            </button>
+            <button
+              onclick={async () => {
+                customFontStatus = null;
+                customFontPathInput = "";
+                customFontFamilyInput = "";
+                try {
+                  await configStore.setCustomFont(null, null);
+                  await applyCustomFont(null, null);
+                  customFontStatus = "Font reset to default.";
+                } catch (e) {
+                  customFontStatus = `Error: ${e instanceof Error ? e.message : String(e)}`;
+                }
+              }}
+              class="px-3 py-2 text-sm rounded-lg border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-secondary)] hover:border-red-400 hover:text-red-400 transition-colors"
+            >
+              Reset
+            </button>
+          </div>
+          {#if customFontStatus}
+            <p class="text-xs text-[var(--text-tertiary)]">{customFontStatus}</p>
+          {/if}
+        </div>
+        <p class="text-xs text-[var(--text-tertiary)] mt-1">
+          Supports Google Fonts URLs, direct font file URLs, or local file paths. Family name is
+          required to apply the font.
+        </p>
+      </div>
+
       <!-- Show Thinking Blocks Toggle -->
       <div class="mb-4">
         <label class="flex items-center gap-3 cursor-pointer">

src/lib/components/InputBar.svelte

@@ -1011,7 +1011,7 @@ User: ${formattedMessage}`;
           : "Connect to Claude first..."}
         disabled={isSubmitting}
         rows={1}
-        style="height: {textareaHeight}px; font-size: var(--terminal-font-size, 14px);"
+        style="height: {textareaHeight}px; font-size: var(--terminal-font-size, 14px); font-family: var(--terminal-font-family, monospace);"
         class="w-full px-4 py-3 bg-[var(--bg-secondary)] border border-[var(--border-color)]
              rounded-lg text-[var(--text-primary)] placeholder-gray-500 resize-none
              input-trans-focus disabled:opacity-50 disabled:cursor-not-allowed"

src/lib/components/StatusBar.svelte

@@ -111,6 +111,8 @@
     trusted_workspaces: [],
     background_image_path: null,
     background_image_opacity: 0.3,
+    custom_font_path: null,
+    custom_font_family: null,
   });
 
   let streamerModeActive = $state(false);

src/lib/components/Terminal.svelte

@@ -231,7 +231,7 @@
     bind:this={terminalElement}
     onscroll={handleScroll}
     class="terminal-content h-[calc(100%-76px)] overflow-y-auto p-4 font-mono"
-    style="font-size: var(--terminal-font-size, 14px);"
+    style="font-size: var(--terminal-font-size, 14px); font-family: var(--terminal-font-family, monospace);"
   >
     {#if lines.length === 0}
       <div class="terminal-waiting italic">

src/lib/stores/config.test.ts

@@ -5,6 +5,7 @@ import {
   maskPaths,
   clampFontSize,
   applyFontSize,
+  applyCustomFont,
   applyTheme,
   applyCustomThemeColors,
   clearCustomThemeColors,
@@ -21,12 +22,17 @@ import {
   type CustomThemeColors,
 } from "./config";
 import { invoke } from "@tauri-apps/api/core";
+import { readFile } from "@tauri-apps/plugin-fs";
 
 // Mock Tauri APIs
 vi.mock("@tauri-apps/api/core", () => ({
   invoke: vi.fn(),
 }));
 
+vi.mock("@tauri-apps/plugin-fs", () => ({
+  readFile: vi.fn(),
+}));
+
 describe("config store", () => {
   describe("font size constants", () => {
     it("has correct MIN_FONT_SIZE", () => {
@@ -206,6 +212,8 @@ describe("config store", () => {
         trusted_workspaces: [],
         background_image_path: null,
         background_image_opacity: 0.3,
+        custom_font_path: null,
+        custom_font_family: null,
       };
 
       expect(config.model).toBe("claude-sonnet-4");
@@ -258,6 +266,8 @@ describe("config store", () => {
         trusted_workspaces: [],
         background_image_path: null,
         background_image_opacity: 0.3,
+        custom_font_path: null,
+        custom_font_family: null,
       };
 
       expect(config.model).toBeNull();
@@ -809,6 +819,8 @@ describe("config store", () => {
         trusted_workspaces: [],
         background_image_path: null,
         background_image_opacity: 0.3,
+        custom_font_path: null,
+        custom_font_family: null,
       };
 
       const mockInvokeImpl = vi.mocked(invoke);
@@ -1133,4 +1145,176 @@ describe("config store", () => {
       expect(get(showThinkingBlocks)).toBe(false);
     });
   });
+
+  describe("applyCustomFont", () => {
+    const readFileMock = vi.mocked(readFile);
+
+    beforeEach(() => {
+      // Remove any style element left by previous tests
+      document.getElementById("hikari-custom-font")?.remove();
+      document.documentElement.style.removeProperty("--terminal-font-family");
+      readFileMock.mockReset();
+    });
+
+    it("removes CSS variable when both path and family are null", async () => {
+      document.documentElement.style.setProperty("--terminal-font-family", "'SomeFont', monospace");
+      await applyCustomFont(null, null);
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe("");
+    });
+
+    it("removes CSS variable when both path and family are empty strings", async () => {
+      document.documentElement.style.setProperty("--terminal-font-family", "'SomeFont', monospace");
+      await applyCustomFont("", "");
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe("");
+      expect(document.getElementById("hikari-custom-font")).toBeNull();
+    });
+
+    it("injects @import for a CSS stylesheet URL", async () => {
+      await applyCustomFont("https://fonts.googleapis.com/css2?family=Fira+Code", "Fira Code");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style).not.toBeNull();
+      expect(style?.textContent).toContain(
+        "@import url('https://fonts.googleapis.com/css2?family=Fira+Code')"
+      );
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe(
+        "'Fira Code', monospace"
+      );
+    });
+
+    it("injects @font-face for a direct font file URL (.woff2)", async () => {
+      await applyCustomFont("https://example.com/fonts/myfont.woff2", "MyFont");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style).not.toBeNull();
+      expect(style?.textContent).toContain("@font-face");
+      expect(style?.textContent).toContain("url('https://example.com/fonts/myfont.woff2')");
+      expect(style?.textContent).toContain("'MyFont'");
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe(
+        "'MyFont', monospace"
+      );
+    });
+
+    it("injects @font-face for a direct font file URL (.ttf)", async () => {
+      await applyCustomFont("https://example.com/fonts/myfont.ttf", "MyTtfFont");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style).not.toBeNull();
+      expect(style?.textContent).toContain("@font-face");
+      expect(style?.textContent).toContain("url('https://example.com/fonts/myfont.ttf')");
+      expect(style?.textContent).toContain("'MyTtfFont'");
+    });
+
+    it("uses HikariCustomFont as fallback family for direct font URLs when family is empty", async () => {
+      await applyCustomFont("https://example.com/fonts/myfont.woff2", "");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style?.textContent).toContain("'HikariCustomFont'");
+    });
+
+    it("reads local file and injects @font-face with data URL", async () => {
+      const fakeData = new Uint8Array([72, 101, 108, 108, 111]); // "Hello"
+      readFileMock.mockResolvedValueOnce(fakeData);
+
+      await applyCustomFont("/home/naomi/.fonts/MyFont.ttf", "MyFont");
+
+      expect(readFileMock).toHaveBeenCalledWith("/home/naomi/.fonts/MyFont.ttf");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style).not.toBeNull();
+      expect(style?.textContent).toContain("@font-face");
+      expect(style?.textContent).toContain("data:font/ttf;base64,");
+      expect(style?.textContent).toContain("'MyFont'");
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe(
+        "'MyFont', monospace"
+      );
+    });
+
+    it("uses HikariCustomFont as fallback family for local files when family is empty", async () => {
+      const fakeData = new Uint8Array([1, 2, 3]);
+      readFileMock.mockResolvedValueOnce(fakeData);
+
+      await applyCustomFont("/home/naomi/.fonts/MyFont.woff2", "");
+
+      const style = document.getElementById("hikari-custom-font");
+      expect(style?.textContent).toContain("'HikariCustomFont'");
+      expect(style?.textContent).toContain("font/woff2");
+    });
+
+    it("sets CSS variable when only family is provided (no path)", async () => {
+      await applyCustomFont("", "SystemFont");
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe(
+        "'SystemFont', monospace"
+      );
+      expect(document.getElementById("hikari-custom-font")).toBeNull();
+    });
+
+    it("replaces a previously injected style element", async () => {
+      await applyCustomFont("https://fonts.googleapis.com/css2?family=Fira+Code", "Fira Code");
+      expect(document.getElementById("hikari-custom-font")).not.toBeNull();
+
+      await applyCustomFont("https://fonts.googleapis.com/css2?family=Roboto+Mono", "Roboto Mono");
+      const styles = document.querySelectorAll("#hikari-custom-font");
+      expect(styles.length).toBe(1);
+      expect(styles[0].textContent).toContain("Roboto+Mono");
+    });
+
+    it("uses correct MIME type for .otf local files", async () => {
+      readFileMock.mockResolvedValueOnce(new Uint8Array([1]));
+      await applyCustomFont("/fonts/MyFont.otf", "OtfFont");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style?.textContent).toContain("font/otf");
+    });
+
+    it("falls back to font/ttf MIME for unknown extension local files", async () => {
+      readFileMock.mockResolvedValueOnce(new Uint8Array([1]));
+      await applyCustomFont("/fonts/MyFont.xyz", "XyzFont");
+      const style = document.getElementById("hikari-custom-font");
+      expect(style?.textContent).toContain("font/ttf");
+    });
+  });
+
+  describe("setCustomFont", () => {
+    const readFileMock = vi.mocked(readFile);
+    const invokeMock = vi.mocked(invoke);
+
+    beforeEach(() => {
+      document.getElementById("hikari-custom-font")?.remove();
+      document.documentElement.style.removeProperty("--terminal-font-family");
+      readFileMock.mockReset();
+      invokeMock.mockResolvedValue(undefined);
+    });
+
+    it("saves config and applies the font", async () => {
+      await configStore.setCustomFont(null, null);
+      await configStore.setCustomFont(
+        "https://fonts.googleapis.com/css2?family=Fira+Code",
+        "Fira Code"
+      );
+
+      expect(invokeMock).toHaveBeenCalledWith(
+        "save_config",
+        expect.objectContaining({
+          config: expect.objectContaining({
+            custom_font_path: "https://fonts.googleapis.com/css2?family=Fira+Code",
+            custom_font_family: "Fira Code",
+          }),
+        })
+      );
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe(
+        "'Fira Code', monospace"
+      );
+    });
+
+    it("clears font when called with nulls", async () => {
+      document.documentElement.style.setProperty("--terminal-font-family", "'SomeFont', monospace");
+      await configStore.setCustomFont(null, null);
+
+      expect(document.documentElement.style.getPropertyValue("--terminal-font-family")).toBe("");
+      expect(invokeMock).toHaveBeenCalledWith(
+        "save_config",
+        expect.objectContaining({
+          config: expect.objectContaining({
+            custom_font_path: null,
+            custom_font_family: null,
+          }),
+        })
+      );
+    });
+  });
 });

src/lib/stores/config.ts

@@ -1,5 +1,6 @@
 import { writable, derived } from "svelte/store";
 import { invoke } from "@tauri-apps/api/core";
+import { readFile } from "@tauri-apps/plugin-fs";
 
 export type Theme = "dark" | "light" | "high-contrast" | "custom";
 export type BudgetAction = "warn" | "block";
@@ -58,6 +59,9 @@ export interface HikariConfig {
   // Background image settings
   background_image_path: string | null;
   background_image_opacity: number;
+  // Custom font settings
+  custom_font_path: string | null;
+  custom_font_family: string | null;
 }
 
 const defaultConfig: HikariConfig = {
@@ -104,6 +108,8 @@ const defaultConfig: HikariConfig = {
   trusted_workspaces: [],
   background_image_path: null,
   background_image_opacity: 0.3,
+  custom_font_path: null,
+  custom_font_family: null,
 };
 
 function createConfigStore() {
@@ -240,6 +246,11 @@ function createConfigStore() {
     setCompactMode: async (enabled: boolean) => {
       await updateConfig({ compact_mode: enabled });
     },
+
+    setCustomFont: async (path: string | null, family: string | null) => {
+      await updateConfig({ custom_font_path: path, custom_font_family: family });
+      await applyCustomFont(path, family);
+    },
   };
 }
 
@@ -305,6 +316,69 @@ export function clampFontSize(size: number): number {
   return Math.max(MIN_FONT_SIZE, Math.min(MAX_FONT_SIZE, size));
 }
 
+const FONT_STYLE_ID = "hikari-custom-font";
+const FONT_CSS_VAR = "--terminal-font-family";
+const DIRECT_FONT_EXTENSIONS = new Set(["woff", "woff2", "ttf", "otf", "eot"]);
+
+export async function applyCustomFont(path: string | null, family: string | null): Promise<void> {
+  if (typeof document === "undefined") return;
+
+  // Remove any previously injected font style
+  document.getElementById(FONT_STYLE_ID)?.remove();
+
+  const trimmedPath = path?.trim() ?? "";
+  const trimmedFamily = family?.trim() ?? "";
+
+  if (!trimmedPath && !trimmedFamily) {
+    document.documentElement.style.removeProperty(FONT_CSS_VAR);
+    return;
+  }
+
+  if (trimmedPath) {
+    const style = document.createElement("style");
+    style.id = FONT_STYLE_ID;
+
+    if (trimmedPath.startsWith("http://") || trimmedPath.startsWith("https://")) {
+      const ext = trimmedPath.split(".").pop()?.toLowerCase() ?? "";
+
+      if (DIRECT_FONT_EXTENSIONS.has(ext)) {
+        // Direct font file URL — inject via @font-face
+        const fontFamily = trimmedFamily || "HikariCustomFont";
+        style.textContent = `@font-face { font-family: '${fontFamily}'; src: url('${trimmedPath}'); }`;
+      } else {
+        // CSS stylesheet URL (e.g. Google Fonts) — inject as @import
+        style.textContent = `@import url('${trimmedPath}');`;
+      }
+    } else {
+      // Local file path — read via Tauri and embed as data URL
+      const data = await readFile(trimmedPath);
+      const chunks: string[] = [];
+      const chunkSize = 8192;
+      for (let i = 0; i < data.length; i += chunkSize) {
+        chunks.push(String.fromCharCode(...data.slice(i, i + chunkSize)));
+      }
+      const ext = trimmedPath.split(".").pop()?.toLowerCase() ?? "ttf";
+      const mimeMap: Record<string, string> = {
+        woff: "font/woff",
+        woff2: "font/woff2",
+        ttf: "font/ttf",
+        otf: "font/otf",
+        eot: "application/vnd.ms-fontobject",
+      };
+      const mime = mimeMap[ext] ?? "font/ttf";
+      const dataUrl = `data:${mime};base64,${btoa(chunks.join(""))}`;
+      const fontFamily = trimmedFamily || "HikariCustomFont";
+      style.textContent = `@font-face { font-family: '${fontFamily}'; src: url('${dataUrl}'); }`;
+    }
+
+    document.head.appendChild(style);
+  }
+
+  if (trimmedFamily) {
+    document.documentElement.style.setProperty(FONT_CSS_VAR, `'${trimmedFamily}', monospace`);
+  }
+}
+
 export { MIN_FONT_SIZE, MAX_FONT_SIZE, DEFAULT_FONT_SIZE };
 
 export const configStore = createConfigStore();

src/routes/+page.svelte

@@ -11,7 +11,13 @@
     updateDiscordRpc,
     setSkipNextGreeting,
   } from "$lib/tauri";
-  import { configStore, applyTheme, applyFontSize, isCompactMode } from "$lib/stores/config";
+  import {
+    configStore,
+    applyTheme,
+    applyFontSize,
+    applyCustomFont,
+    isCompactMode,
+  } from "$lib/stores/config";
   import { readFile } from "@tauri-apps/plugin-fs";
   import { initNotificationSync, cleanupNotificationSync } from "$lib/stores/notifications";
   import { conversationsStore } from "$lib/stores/conversations";
@@ -454,6 +460,7 @@
       const config = configStore.getConfig();
       applyTheme(config.theme, config.custom_theme_colors);
       applyFontSize(config.font_size);
+      await applyCustomFont(config.custom_font_path, config.custom_font_family);
 
       // Apply always-on-top setting
       if (config.always_on_top) {