feat(ui): changed Background color, centered Lobby

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

knockoutwhistweb/app/views/ingame/ingame.scala.html

@@ -3,6 +3,7 @@
 @(player: de.knockoutwhist.player.AbstractPlayer, gamelobby: logic.game.GameLobby)
 
 @main("Ingame") {
+<main class="game-field-background vh-100">
     <div class="py-5 ms-4 me-4">
 
         <div class="row ms-4 me-4">
@@ -67,4 +68,5 @@
             </div>
         </div>
     </div>
+</main>
 }