Structure legacy-compatible frontend CSS

app/game-frontend/src/assets/main.css

@@ -1,4 +1,7 @@
 @import 'tailwindcss';
+@import './styles/tokens.css';
+@import './styles/game-shell.css';
+@import './styles/ref-shell.css';
 
 @theme {
     --color-sammo-ink: #000;
@@ -15,7 +18,7 @@ body {
     margin: 0;
     background: #000;
     color: #fff;
-    font-family: Pretendard, 'Apple SD Gothic Neo', 'Noto Sans KR', 'Malgun Gothic', sans-serif;
+    font-family: var(--sammo-font-sans);
     font-size: 14px;
     line-height: 1.3;
 }
@@ -29,17 +32,17 @@ textarea {
 
 .legacy-bg0 {
     background-color: #302016;
-    background-image: url('/image/game/back_walnut.jpg');
+    background-image: var(--sammo-texture-walnut);
 }
 
 .legacy-bg1 {
     background-color: #14241b;
-    background-image: url('/image/game/back_green.jpg');
+    background-image: var(--sammo-texture-green);
 }
 
 .legacy-bg2 {
     background-color: #172a52;
-    background-image: url('/image/game/back_blue.jpg');
+    background-image: var(--sammo-texture-blue);
 }
 
 .legacy-button {