feat: refactor theme management and enhance color customization

- Cleaned up theme architecture by consolidating CSS variables and removing redundant theme applications, ensuring a single source of truth for theming.
- Implemented a dark mode override and improved color management using CSS variables for better customization.
- Updated various components to utilize new color variables, enhancing maintainability and visual consistency across the application.
- Added detailed tasks in TODO.md for future enhancements related to user preferences and color customization features.

src/app/jira-dashboard/JiraDashboardPageClient.tsx

@@ -262,7 +262,7 @@ export function JiraDashboardPageClient({ initialJiraConfig, initialAnalytics }:
           {error && (
             <Card className="mb-6 border-red-500/20 bg-red-500/10">
               <CardContent className="p-4">
-                <div className="flex items-center gap-2 text-red-600 dark:text-red-400">
+                <div className="flex items-center gap-2" style={{ color: 'var(--destructive)' }}>
                   <span>❌</span>
                   <span>{error}</span>
                 </div>
@@ -273,7 +273,7 @@ export function JiraDashboardPageClient({ initialJiraConfig, initialAnalytics }:
           {exportError && (
             <Card className="mb-6 border-orange-500/20 bg-orange-500/10">
               <CardContent className="p-4">
-                <div className="flex items-center gap-2 text-orange-600 dark:text-orange-400">
+                <div className="flex items-center gap-2" style={{ color: 'var(--accent)' }}>
                   <span>⚠️</span>
                   <span>Erreur d&apos;export: {exportError}</span>
                 </div>