Fix the key mismatch to solve FOUC.

lib/decentralised_book_index_web/components/layouts/root.html.heex

@@ -12,7 +12,7 @@
     </script>
     <script>
       // On page load or when changing themes, best to add inline in `head` to avoid FOUC
-      if (localStorage.getItem('color-theme') === 'dark' || (!('color-theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
+      if (localStorage.getItem('theme') === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
         document.documentElement.classList.add('dark');
       } else {
         document.documentElement.classList.remove('dark')