feat(styles): migrate design system tokens to CSS variables

Replace minimal background/foreground vars with a comprehensive design
system covering colors (background, text, brand, profile, gifts,
activities, nav), spacing, and border-radius tokens. Mirrors the
original Dart design constants under src/design/*.ts and supports
dark mode.

barrelsby.json

@@ -10,6 +10,7 @@
     "./src/data/storage/auth",
     "./src/data/storage/user",
     "./src/data/storage/chat",
-    "./src/data/storage/app"
+    "./src/data/storage/app", 
+    "./src/design"
   ]
 }