wp-includes/css/customize-preview.css

   1 .customize-partial-refreshing {
   2         opacity: 0.25;
   3         -webkit-transition: opacity 0.25s;
   4         transition: opacity 0.25s;
   5         cursor: progress;
   6 }
   7
   8 /* Override highlight when refreshing */
   9 .customize-partial-refreshing.widget-customizer-highlighted-widget {
  10         -webkit-box-shadow: none;
  11         box-shadow: none;
  12 }