feat(ui): changed Background color, centered Lobby

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

knockoutwhistweb/app/assets/stylesheets/main.less

@@ -14,7 +14,7 @@
   --bs-border-color: rgba(0, 0, 0, 0.125) !important;
   --bs-heading-color: var(--color) !important;
 }
-
+@background-color: var(--background-color);
 @highlightcolor: var(--highlightscolor);
 @background-image: var(--background-image);
 @color: var(--color);
@@ -29,6 +29,10 @@
     background-repeat: no-repeat;
     background-attachment: fixed;
 }
+.lobby-background {
+    background-color: @background-color;
+
+}
 
 .navbar-header{
     text-align:center;