Move common css props to mixin

app/assets/stylesheets/player/player.css.scss

@@ -14,6 +14,8 @@ $color5: #EDC951;
 @mixin player-overlay {
   background-color: rgba(0, 0, 0, 0.5);
   z-index: 10;
+  background-repeat: no-repeat;
+  background-position: center;
   position: absolute;
   top: 0;
   left: 0;
@@ -69,12 +71,12 @@ $color5: #EDC951;
   .loading {
     @include player-overlay;
     background-image: url(image-path('loader.gif'));
-    background-repeat: no-repeat;
-    background-position: center;
   }
 
   .start-prompt {
     @include player-overlay;
+    background-image: url(image-path('play-button-1.png'));
+    z-index: 20;
     cursor: pointer;
 
     .play-button {