chrome/renderer/resources/extensions/window_controls_template.html

<template id="window-controls-template">
  <style>
    .controls {
      width:32px;
      height:32px;
      position:absolute;
      z-index:200;
    }
    #close-control {
      top:8px;
      right:10px;
    }
    #maximize-control {
      top:8px;
      right:52px;
    }
    #close {
      top:0;
      right:0;
      -webkit-mask-image:url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAALNJREFUeNrsllEOwyAIhmVXwIu487dXKEdy2PBgWuxwm/VhkPwvSuALAhFyzmGmPcJkcwAHcAAHMAMAQGItLGSFhlB8kpmgrGKL2NbiziIWKqHK2SY+qzluBwBKcg2iTr7fjQBoQZySd1W2E0CD2LSqjAQ4Qqh9YY37zRj+5iPx4RPUZac7n6DVhHRHE74bQxo9hvUiio1FRCMXURKIeNFSKD5Pa1zwX7EDOIAD/D3AS4ABAKWdkCCeGGsrAAAAAElFTkSuQmCC');
    }
    .windowbutton {
      width:32px;
      height:32px;
      position:absolute;
      background-color:rgba(0, 0, 0, 0.49);
    }
    .windowbuttonbackground {
      width:32px;
      height:32px;
      position:absolute;
    }
    .windowbuttonbackground:hover {
      background-image:url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAEBJREFUeNrszrENAEAIw8A8EvU32X9RGsQUaewFfM/2l9TKNBWcX10KBwAAAAAAAAAAAAAAAAAAABxggv9ZAQYAhakDi3I15kgAAAAASUVORK5CYII=');

    }
    .windowbuttonbackground:active {
      background-image:url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAFtJREFUeNrs1zESgCAMRNEAkSZD5VW8/1k8hFaCCqfY5u9M6v/apIh2mH1hkqXba92uUvxU5Mfoe57xx0Rb7WziAQAAAAAAAAAAAAAAAAAAOcDXkzqvifrvL8AAWBcLpapo5CcAAAAASUVORK5CYII=');
    }
    #maximize {
      -webkit-mask-image:url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAFFJREFUeNrs1jsOACAIREHWeP8royWdnwaDj9pi2OhGubtlTrPkAQAAAIC+e1DSUWXOhlWtBGIYq+W5hAAA1GzC23f+fALiVwwAAIDvAUOAAQAv/Aw+jTHzugAAAABJRU5ErkJggg==');
    }
  </style>
  <div id="close-control" class="controls">
    <div id="close" class="windowbutton"> </div>
    <div id="close-background" class="windowbuttonbackground"> </div>
  </div>
  <div id="maximize-control" class="controls">
    <div id="maximize" class="windowbutton"></div>
    <div id="maximize-background" class="windowbuttonbackground"></div>
  </div>
</template>