💄 Replace css files with scss and factorize CSS rules

frontend/src/index.tsx

@@ -1,7 +1,7 @@
 /* @refresh reload */
 import { render } from 'solid-js/web';
 
-import './index.css';
+import './index.scss';
 import App from './App';
 
 render(() => (<App/>), document.getElementById('root') as HTMLElement);