feat(ui): changed Background color, centered Lobby

Added a background color for the mainmenu and the lobby + centered lobby

knockoutwhistweb/app/views/main.scala.html

@@ -22,6 +22,7 @@
             @* And here's where we render the `Html` object containing
              * the page content. *@
             @content
+
         <script src="@routes.JavaScriptRoutingController.javascriptRoutes()" type="text/javascript"></script>
         <script src="@routes.Assets.versioned("javascripts/main.js")" type="text/javascript"></script>
         <script src="https://cdn.jsdelivr.net/npm/bootstrap@@5.3.8/dist/js/bootstrap.bundle.min.js" integrity="sha384-FKyoEForCGlyvwx9Hj09JcYn3nv7wiPVlz7YYwJrWVcXK/BmnVDxM+D2scQbITxI" crossorigin="anonymous"></script>

knockoutwhistweb/app/views/mainmenu/creategame.scala.html

@@ -4,7 +4,7 @@
 @navbar(user)
 <main class="lobby-background flex-grow-1">
     <div class="w-50 mx-auto">
-        <div class="mt-3">
+        <div class="w-50 mx-auto">
             <label for="lobbyname" class="form-label">Lobby-Name</label>
             <input type="text" class="form-control" id="lobbyname" name="lobbyname" placeholder="Lobby 1" required>
         </div>