💄 Change background colours (#22)

### 📝 작업 내용

- 헤더의 배경색을 회색으로, 콘텐츠 영역의 배경색을 흰색으로 바꾸어 보았습니다.

### 📸 스크린샷 (선택)

<img width="1806" height="1168" alt="스크린샷 2026-01-14 05 50 02"
src="https://github.com/user-attachments/assets/bc14f4cd-ceae-46dc-af3e-d87b1750cd11"
/>

### 🚀 리뷰 요구사항 (선택)

- 한번 살펴보시고 괜찮은지 판단을 부탁드립니다.
- 괜찮으면 병합을 눌러주시고, 아닌 것 같다면 이 PR를 닫아 주세요.

Author: young-52

src/components/Header.tsx

@@ -15,12 +15,12 @@ export default function Header() {
   const { isLoggedIn, handleLogout } = useAuth();
 
   const linkClassName = (isActive: boolean) => `
-    font-bold h-[42px] items-center px-4 py-2 rounded-md
-    ${isActive ? 'bg-gray-200' : 'hover:bg-gray-200 transition-all text-gray-500'}
+    h-[42px] items-center px-4 py-2 rounded-md text-black font-semibold
+    ${isActive ? 'bg-gray-200' : 'hover:bg-gray-200 transition-all'}
   `;
 
   return (
-    <header className="sticky top-0 z-[100] flex w-full justify-center bg-white shadow-md">
+    <header className="sticky top-0 z-[100] flex w-full justify-center bg-gray-50 border">
       <div className="flex w-full items-center justify-between px-6 py-4 sm:w-screen-sm md:w-screen-md lg:w-screen-lg xl:max-w-screen-xl">
         <Link to="/" className="text-xl font-bold font-title">
           모이샤

src/layouts/RootLayout.tsx

@@ -7,7 +7,7 @@ export default function RootLayout() {
     <div className="flex flex-col min-h-screen">
       <Header />
       <main
-        className="flex w-full flex-1 flex-col items-center justify-center bg-gray-50 px-3 xs:px-0"
+        className="flex w-full flex-1 flex-col items-center justify-center bg-white px-3 xs:px-0"
         style={{ padding: '2rem' }}
       >
         <Outlet />