feat: implement comprehensive theming system

- Add ThemeContext with support for dark, gray, light, and custom themes
- Create theme switching UI in Settings with theme selector
- Add custom color editor for custom theme mode
- Update styles.css with theme-specific CSS variables
- Add theme storage API methods for persistence
- Update syntax highlighting to match selected theme
- Wrap App with ThemeProvider for global theme access

The theming system allows users to switch between predefined themes
or create their own custom theme with live color editing.

src/hooks/useTheme.ts

@@ -0,0 +1,24 @@
+import { useThemeContext } from '../contexts/ThemeContext';
+
+/**
+ * Hook to access and control the theme system
+ * 
+ * @returns {Object} Theme utilities and state
+ * @returns {ThemeMode} theme - Current theme mode ('dark' | 'gray' | 'light' | 'custom')
+ * @returns {CustomThemeColors} customColors - Custom theme color configuration
+ * @returns {Function} setTheme - Function to change the theme mode
+ * @returns {Function} setCustomColors - Function to update custom theme colors
+ * @returns {boolean} isLoading - Whether theme operations are in progress
+ * 
+ * @example
+ * const { theme, setTheme } = useTheme();
+ * 
+ * // Change theme
+ * await setTheme('light');
+ * 
+ * // Update custom colors
+ * await setCustomColors({ background: 'oklch(0.98 0.01 240)' });
+ */
+export const useTheme = () => {
+  return useThemeContext();
+};