Update diagonal cell styling in TournamentsView for improved visibility

Modified the CSS for diagonal cells in TournamentsView to change the background and text color from black to a lighter gray. This enhancement improves the visual clarity of the tournament display, making it easier for users to identify active matches.

frontend/src/views/TournamentsView.vue

@@ -1780,8 +1780,8 @@ button {
 }
 
 .diagonal {
-    background-color: #000;
-    color: #000;
+    background-color: #bbb;
+    color: #bbb;
     display: block;
     position: absolute;
     top: 0;