refactor: use tailwind custom colors for themes

Use tailwind v4 @theme block so we can use bg-layer-0 instead of
bg-[--layer-0] for theme colors.

app/src/app.css

@@ -4,3 +4,7 @@
   prefix: "i";
   icon-sets: from-folder(custom, "./src/lib/icons");
 }
+
+body * {
+  color: var(--color-text);
+}