Chore: move custom css loading, add letter-spacing (#4359)

src/pages/_document.jsx

@@ -10,6 +10,8 @@ export default function Document() {
         />
         <meta name="mobile-web-app-capable" content="yes" />
         <link rel="manifest" href="/site.webmanifest?v=4" crossOrigin="use-credentials" />
+        <link rel="preload" href="/api/config/custom.css" as="style" />
+        <link rel="stylesheet" href="/api/config/custom.css" /> {/* eslint-disable-line @next/next/no-css-tags */}
       </Head>
       <body>
         <Main />