panels: Adjust opacity value for the exit widget.

This makes this icon less invisible.

Once the user hovers over the "x" it will become brighter to notify the user.

static/styles/night_mode.css

@@ -771,6 +771,11 @@ on a dark background, and don't change the dark labels dark either. */
 
         .close {
             color: inherit;
+            opacity: 0.8;
+        }
+
+        .close:hover {
+            opacity: 1;
         }
     }