feat: add custom theme support with color picker UI

- Add Custom variant to Theme enum and CustomThemeColors struct in Rust config
- Add custom_theme_colors field to HikariConfig for storing user-defined colors
- Create color picker UI in ConfigSidebar with 8 customizable color variables
- Implement live preview when editing custom theme colors
- Apply custom colors on app startup when custom theme is selected
- Use dark theme as base, override with user's custom CSS variables

src-tauri/src/config.rs

@@ -92,6 +92,10 @@ pub struct HikariConfig {
 
     #[serde(default)]
     pub profile_bio: Option<String>,
+
+    // Custom theme colors
+    #[serde(default)]
+    pub custom_theme_colors: CustomThemeColors,
 }
 
 impl Default for HikariConfig {
@@ -118,6 +122,7 @@ impl Default for HikariConfig {
             profile_name: None,
             profile_avatar_path: None,
             profile_bio: None,
+            custom_theme_colors: CustomThemeColors::default(),
         }
     }
 }
@@ -150,6 +155,27 @@ pub enum Theme {
     Light,
     #[serde(rename = "high-contrast")]
     HighContrast,
+    Custom,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
+pub struct CustomThemeColors {
+    #[serde(default)]
+    pub bg_primary: Option<String>,
+    #[serde(default)]
+    pub bg_secondary: Option<String>,
+    #[serde(default)]
+    pub bg_terminal: Option<String>,
+    #[serde(default)]
+    pub accent_primary: Option<String>,
+    #[serde(default)]
+    pub accent_secondary: Option<String>,
+    #[serde(default)]
+    pub text_primary: Option<String>,
+    #[serde(default)]
+    pub text_secondary: Option<String>,
+    #[serde(default)]
+    pub border_color: Option<String>,
 }
 
 #[cfg(test)]
@@ -178,6 +204,7 @@ mod tests {
         assert!(config.profile_name.is_none());
         assert!(config.profile_avatar_path.is_none());
         assert!(config.profile_bio.is_none());
+        assert_eq!(config.custom_theme_colors, CustomThemeColors::default());
     }
 
     #[test]
@@ -204,6 +231,7 @@ mod tests {
             profile_name: Some("Test User".to_string()),
             profile_avatar_path: None,
             profile_bio: Some("A test bio".to_string()),
+            custom_theme_colors: CustomThemeColors::default(),
         };
 
         let json = serde_json::to_string(&config).unwrap();
@@ -232,5 +260,8 @@ mod tests {
             serde_json::to_string(&high_contrast).unwrap(),
             "\"high-contrast\""
         );
+
+        let custom = Theme::Custom;
+        assert_eq!(serde_json::to_string(&custom).unwrap(), "\"custom\"");
     }
 }