Implement theme switching and enhance dark mode support (#43)

frontend/src/contexts/ThemeContext.tsx

@@ -0,0 +1,76 @@
+import React, { createContext, useContext, useState, useEffect, ReactNode } from 'react';
+
+type Theme = 'light' | 'dark' | 'system';
+
+interface ThemeContextType {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  resolvedTheme: 'light' | 'dark'; // The actual theme used after resolving system preference
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+export const useTheme = () => {
+  const context = useContext(ThemeContext);
+  if (!context) {
+    throw new Error('useTheme must be used within a ThemeProvider');
+  }
+  return context;
+};
+
+export const ThemeProvider: React.FC<{ children: ReactNode }> = ({ children }) => {
+  // Get theme from localStorage or default to 'system'
+  const [theme, setTheme] = useState<Theme>(() => {
+    const savedTheme = localStorage.getItem('theme') as Theme;
+    return savedTheme || 'system';
+  });
+  
+  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');
+
+  // Function to set theme and save to localStorage
+  const handleSetTheme = (newTheme: Theme) => {
+    setTheme(newTheme);
+    localStorage.setItem('theme', newTheme);
+  };
+
+  // Effect to handle system theme changes and apply theme to document
+  useEffect(() => {
+    const updateTheme = () => {
+      const root = window.document.documentElement;
+      const systemTheme = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+      
+      // Determine which theme to use
+      const themeToApply = theme === 'system' ? systemTheme : theme;
+      setResolvedTheme(themeToApply as 'light' | 'dark');
+      
+      // Apply or remove dark class based on theme
+      if (themeToApply === 'dark') {
+        console.log('Applying dark mode to HTML root element'); // 添加日志
+        root.classList.add('dark');
+        document.body.style.backgroundColor = '#111827'; // Force a dark background to ensure visible effect
+      } else {
+        console.log('Removing dark mode from HTML root element'); // 添加日志
+        root.classList.remove('dark');
+        document.body.style.backgroundColor = ''; // Reset background color
+      }
+    };
+
+    // Set up listeners for system theme changes
+    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+    mediaQuery.addEventListener('change', updateTheme);
+    
+    // Initial theme setup
+    updateTheme();
+
+    // Cleanup
+    return () => {
+      mediaQuery.removeEventListener('change', updateTheme);
+    };
+  }, [theme]);
+
+  return (
+    <ThemeContext.Provider value={{ theme, setTheme: handleSetTheme, resolvedTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+};