Remove background color from overlay cast button

Source/core/css/mediaControls.css

 /*
  * Copyright (C) 2009 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 143 matching lines @@
 video::-internal-media-controls-overlay-cast-button {
     -webkit-appearance: -internal-media-overlay-cast-off-button;
     display: flex;
     position: absolute;
     top: 5%;
     left: 5%;
     margin-left: 0px;
     margin-top: 0px;
     border: none;
     box-sizing: border-box;
-    background-color: rgba(0,0,0,0.5);
+    background-color: transparent;
     width: 30px;
     height: 30px;
     padding: 0;
 }
 
 audio::-webkit-media-controls-play-button, video::-webkit-media-controls-play-button {
     -webkit-appearance: media-play-button;
     display: flex;
     flex: none;
     border: none;
@@ skipping 237 matching lines @@
     font-weight: bold;
 }
 
 video::-webkit-media-text-track-container u {
     text-decoration: underline;
 }
 
 video::-webkit-media-text-track-container i {
     font-style: italic;
 }