feat: 라이트/다크 모드 - 팝업 메뉴 및 핵심 색상 변경

- 테마 토글을 팝업 메뉴로 변경 (시스템/다크/라이트 선택)
- App.jsx: 모든 레이아웃을 CSS 변수로 변경
- Sidebar.jsx: 배경색, 프로필 메뉴, 드롭다운을 CSS 변수로 변경
- 라이트 모드에서 기본 배경/텍스트 색상 적용

frontend/src/App.jsx

@@ -49,7 +49,7 @@ function App() {
     return (
       <ThemeProvider>
       <DeviceLayout>
-        <div className="min-h-screen bg-mc-bg text-gray-200 font-sans selection:bg-mc-green/30">
+        <div className="min-h-screen bg-[var(--bg-primary)] text-[var(--text-primary)] font-sans selection:bg-mc-green/30">
           <div className="absolute inset-0 bg-[url('https://www.transparenttextures.com/patterns/cubes.png')] opacity-[0.03] pointer-events-none"></div>
           <AnimatePresence mode="wait">
             <Routes location={location} key={location.pathname}>
@@ -69,7 +69,7 @@ function App() {
     return (
       <ThemeProvider>
       <DeviceLayout>
-        <div className="bg-mc-bg text-gray-200 font-sans selection:bg-mc-green/30">
+        <div className="bg-[var(--bg-primary)] text-[var(--text-primary)] font-sans selection:bg-mc-green/30">
           <div className="absolute inset-0 bg-[url('https://www.transparenttextures.com/patterns/cubes.png')] opacity-[0.03] pointer-events-none"></div>
           <AnimatePresence mode="wait">
             <Routes location={location} key={location.pathname}>
@@ -86,7 +86,7 @@ function App() {
   return (
     <ThemeProvider>
     <DeviceLayout>
-      <div className="min-h-screen bg-mc-bg text-gray-200 font-sans selection:bg-mc-green/30">
+      <div className="min-h-screen bg-[var(--bg-primary)] text-[var(--text-primary)] font-sans selection:bg-mc-green/30">
         <div className="absolute inset-0 bg-[url('https://www.transparenttextures.com/patterns/cubes.png')] opacity-[0.03] pointer-events-none"></div>
         
         {/* 사이드바 + 메인 콘텐츠 레이아웃 */}

frontend/src/components/Sidebar.jsx

@@ -175,10 +175,10 @@ const Sidebar = ({ isMobile = false }) => {
           animate={{ opacity: 1, y: 0, scale: 1 }}
           exit={{ opacity: 0, y: -10, scale: 0.95 }}
           transition={{ duration: 0.15 }}
-          className="absolute bottom-full left-0 right-0 mb-2 bg-zinc-800 border border-zinc-700 rounded-xl shadow-xl overflow-hidden z-50"
+          className="absolute bottom-full left-0 right-0 mb-2 bg-[var(--bg-secondary)] border border-[var(--border-color)] rounded-xl shadow-xl overflow-hidden z-50"
         >
           {/* 프로필 정보 */}
-          <div className="p-4 bg-zinc-800/80 border-b border-zinc-700">
+          <div className="p-4 bg-[var(--bg-tertiary)] border-b border-[var(--border-color)]">
             <div className="flex items-center gap-3">
               <img
                 src={user?.profileUrl || '/default-profile.png'}
@@ -202,7 +202,7 @@ const Sidebar = ({ isMobile = false }) => {
           <div className="py-1">
             <button
               onClick={() => { setShowProfileMenu(false); navigate('/profile'); }}
-              className="w-full flex items-center gap-3 px-4 py-2.5 text-sm text-zinc-300 hover:bg-zinc-700 hover:text-white transition-colors"
+              className="w-full flex items-center gap-3 px-4 py-2.5 text-sm text-[var(--text-secondary)] hover:bg-[var(--bg-hover)] hover:text-[var(--text-primary)] transition-colors"
             >
               <User size={16} />
               프로필 관리
@@ -290,8 +290,24 @@ const Sidebar = ({ isMobile = false }) => {
     </AnimatePresence>
   );
 
-  // 테마 토글 버튼 컴포넌트
+  // 테마 선택 메뉴 컴포넌트
+  const [showThemeMenu, setShowThemeMenu] = useState(false);
+  const themeMenuRef = useRef(null);
+  
+  // 테마 메뉴 외부 클릭 시 닫기
+  useEffect(() => {
+    const handleClickOutside = (e) => {
+      if (themeMenuRef.current && !themeMenuRef.current.contains(e.target)) {
+        setShowThemeMenu(false);
+      }
+    };
+    document.addEventListener('mousedown', handleClickOutside);
+    return () => document.removeEventListener('mousedown', handleClickOutside);
+  }, []);
+
   const ThemeToggle = () => {
+    const { setTheme } = useTheme();
+    
     const getThemeIcon = () => {
       if (theme === 'system') return <Monitor size={18} />;
       if (theme === 'dark') return <Moon size={18} />;
@@ -303,16 +319,56 @@ const Sidebar = ({ isMobile = false }) => {
       if (theme === 'dark') return '다크';
       return '라이트';
     };
+
+    const themeOptions = [
+      { value: 'system', label: '시스템', icon: <Monitor size={16} /> },
+      { value: 'dark', label: '다크', icon: <Moon size={16} /> },
+      { value: 'light', label: '라이트', icon: <Sun size={16} /> },
+    ];
     
     return (
-      <button
+      <div className="relative px-4 mb-2" ref={themeMenuRef}>
-        onClick={cycleTheme}
+        <button
-        className="w-full flex items-center gap-3 px-4 py-3 mx-4 mb-2 rounded-xl text-[var(--text-secondary)] hover:text-[var(--text-primary)] hover:bg-[var(--bg-hover)] transition-colors"
+          onClick={() => setShowThemeMenu(!showThemeMenu)}
-        style={{ width: 'calc(100% - 32px)' }}
+          className="w-full flex items-center gap-3 px-4 py-3 rounded-xl text-[var(--text-secondary)] hover:text-[var(--text-primary)] hover:bg-[var(--bg-hover)] transition-colors"
-      >
+        >
-        {getThemeIcon()}
+          {getThemeIcon()}
-        <span className="font-medium">테마: {getThemeLabel()}</span>
+          <span className="font-medium">테마: {getThemeLabel()}</span>
-      </button>
+        </button>
+        
+        <AnimatePresence>
+          {showThemeMenu && (
+            <motion.div
+              initial={{ opacity: 0, y: 10, scale: 0.95 }}
+              animate={{ opacity: 1, y: 0, scale: 1 }}
+              exit={{ opacity: 0, y: 10, scale: 0.95 }}
+              transition={{ duration: 0.15 }}
+              className="absolute bottom-full left-4 right-4 mb-2 bg-[var(--bg-secondary)] border border-[var(--border-color)] rounded-xl shadow-xl overflow-hidden z-50"
+            >
+              {themeOptions.map((option) => (
+                <button
+                  key={option.value}
+                  onClick={() => {
+                    setTheme(option.value);
+                    setShowThemeMenu(false);
+                  }}
+                  className={`w-full flex items-center gap-3 px-4 py-2.5 text-sm transition-colors ${
+                    theme === option.value
+                      ? 'text-mc-green bg-mc-green/10'
+                      : 'text-[var(--text-secondary)] hover:bg-[var(--bg-hover)] hover:text-[var(--text-primary)]'
+                  }`}
+                >
+                  {option.icon}
+                  {option.label}
+                  {theme === option.value && (
+                    <span className="ml-auto text-mc-green">✓</span>
+                  )}
+                </button>
+              ))}
+            </motion.div>
+          )}
+        </AnimatePresence>
+      </div>
     );
   };
 
@@ -358,7 +414,7 @@ const Sidebar = ({ isMobile = false }) => {
           )}
         </AnimatePresence>
         {/* 상단 툴바 */}
-        <header className="fixed top-0 left-0 right-0 z-50 bg-mc-dark/95 backdrop-blur-xl border-b border-white/10 safe-area-top">
+        <header className="fixed top-0 left-0 right-0 z-50 bg-[var(--bg-secondary)]/95 backdrop-blur-xl border-b border-white/10 safe-area-top">
           <div className="flex items-center justify-between h-14 px-4">
             {/* 햄버거 메뉴 버튼 */}
             <button
@@ -394,10 +450,10 @@ const Sidebar = ({ isMobile = false }) => {
                       animate={{ opacity: 1, y: 0, scale: 1 }}
                       exit={{ opacity: 0, y: -10, scale: 0.95 }}
                       transition={{ duration: 0.15 }}
-                      className="absolute top-full right-0 mt-2 w-56 bg-zinc-800 border border-zinc-700 rounded-xl shadow-xl overflow-hidden z-[60]"
+                      className="absolute top-full right-0 mt-2 w-56 bg-[var(--bg-secondary)] border border-[var(--border-color)] rounded-xl shadow-xl overflow-hidden z-[60]"
                     >
                       {/* 프로필 정보 */}
-                      <div className="p-3 bg-zinc-800/80 border-b border-zinc-700">
+                      <div className="p-3 bg-[var(--bg-tertiary)] border-b border-[var(--border-color)]">
                         <div className="flex items-center gap-3">
                           <img
                             src={user?.profileUrl || '/default-profile.png'}
@@ -416,7 +472,7 @@ const Sidebar = ({ isMobile = false }) => {
                       <div className="py-1">
                         <button
                           onClick={() => { setShowProfileMenu(false); navigate('/profile'); }}
-                          className="w-full flex items-center gap-3 px-4 py-2.5 text-sm text-zinc-300 hover:bg-zinc-700 hover:text-white transition-colors"
+                          className="w-full flex items-center gap-3 px-4 py-2.5 text-sm text-[var(--text-secondary)] hover:bg-[var(--bg-hover)] hover:text-[var(--text-primary)] transition-colors"
                         >
                           <User size={16} />
                           프로필 수정
@@ -489,7 +545,7 @@ const Sidebar = ({ isMobile = false }) => {
               animate={{ x: 0 }}
               exit={{ x: -280 }}
               transition={{ type: 'tween', duration: 0.25, ease: 'easeOut' }}
-              className="fixed left-0 top-0 bottom-0 w-72 bg-mc-dark/98 backdrop-blur-xl border-r border-white/10 z-[70] flex flex-col safe-area-left"
+              className="fixed left-0 top-0 bottom-0 w-72 bg-[var(--bg-secondary)]/98 backdrop-blur-xl border-r border-white/10 z-[70] flex flex-col safe-area-left"
             >
               {/* 사이드바 헤더 */}
               <div className="flex items-center justify-between p-4 border-b border-white/10">
@@ -571,7 +627,7 @@ const Sidebar = ({ isMobile = false }) => {
         )}
       </AnimatePresence>
       {/* 데스크탑 사이드바 (항상 표시) */}
-      <aside className="hidden md:flex fixed left-0 top-0 bottom-0 w-64 bg-mc-dark/95 backdrop-blur-xl border-r border-white/10 z-30 flex-col">
+      <aside className="hidden md:flex fixed left-0 top-0 bottom-0 w-64 bg-[var(--bg-secondary)]/95 backdrop-blur-xl border-r border-white/10 z-30 flex-col">
         {/* 로고 */}
         <div className="p-6 border-b border-white/10">
           <div className="flex items-center gap-3">