feat(ui): changed Background color, centered Lobby

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

knockoutwhistweb/app/assets/stylesheets/dark-mode.less

@@ -3,7 +3,7 @@
     --background-image: url('/assets/images/background.png') !important;
     --color: #f8f9fa !important; /* Light text on dark bg */
     --highlightscolor: rgba(131, 131, 131, 0.75) !important;
-
+    --background-color: #192734;
     /* Bootstrap variable overrides for dark mode */
     --bs-body-color: var(--color);
     --bs-link-color: #66b2ff;