feat: implement theme system and UI updates

- Added theme context and provider for light/dark mode support.
- Integrated theme toggle button in the Header component.
- Updated UI components to utilize CSS variables for consistent theming.
- Enhanced Kanban components and forms with new theme styles for better visual coherence.
- Adjusted global styles to define color variables for both themes, improving maintainability.

src/contexts/ThemeContext.tsx

@@ -0,0 +1,71 @@
+'use client';
+
+import { createContext, useContext, useEffect, useState, ReactNode } from 'react';
+
+type Theme = 'light' | 'dark';
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme: () => void;
+  setTheme: (theme: Theme) => void;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+interface ThemeProviderProps {
+  children: ReactNode;
+}
+
+export function ThemeProvider({ children }: ThemeProviderProps) {
+  const [theme, setThemeState] = useState<Theme>('dark');
+  const [mounted, setMounted] = useState(false);
+
+  // Hydration safe initialization
+  useEffect(() => {
+    setMounted(true);
+    
+    // Check localStorage first
+    const savedTheme = localStorage.getItem('theme') as Theme | null;
+    if (savedTheme) {
+      setThemeState(savedTheme);
+      return;
+    }
+
+    // Fallback to system preference
+    if (window.matchMedia('(prefers-color-scheme: light)').matches) {
+      setThemeState('light');
+    }
+  }, []);
+
+  // Apply theme class to document
+  useEffect(() => {
+    if (mounted) {
+      document.documentElement.className = theme;
+      localStorage.setItem('theme', theme);
+    }
+  }, [theme, mounted]);
+
+  const toggleTheme = () => {
+    setThemeState(prev => prev === 'dark' ? 'light' : 'dark');
+  };
+
+  const setTheme = (newTheme: Theme) => {
+    setThemeState(newTheme);
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme, setTheme }}>
+      <div className={mounted ? theme : 'dark'}>
+        {children}
+      </div>
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (context === undefined) {
+    throw new Error('useTheme must be used within a ThemeProvider');
+  }
+  return context;
+}