Update background color and width for the page content.

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

@@ -2,8 +2,8 @@
   <.partial_navbar page_title={assigns[:page_title]} />
 </header>
 
-<main class="px-4 py-20 sm:px-6 lg:px-8">
+<main class="px-4 pt-20 pb-2 sm:px-6 lg:px-8 dark:bg-gray-900">
-  <div class="mx-auto max-w-2xl">
+  <div class="mx-auto max-w-4xl">
     <.flash_group flash={@flash} />
     {@inner_content}
   </div>