<div class="window">
  <div class="window__bar">Title</div>
  <div class="window__content-frame">
    <div class="window__content">
    </div>
  </div>
</div>
$border-radius: 6px;
$btn-radius: 7px;

* {
  box-sizing: border-box;
}
body {
  background: linear-gradient(to right, #936b78 0%, #6a6f91 100%);
  padding: 20px;
}

.window {
  font-family: -apple-system,
	    BlinkMacSystemFont,
	    "Helvetica Neue",
	    sans-serif;
  border-top-left-radius: $border-radius;
  border-top-right-radius: $border-radius;
  border: 1px solid #adadad;
  max-width: 500px;
  box-shadow: 0 20px 70px rgba(0, 0, 0, 0.55);
  margin: 0 auto;
  
  &__bar {
    border: 1px solid white;
    border-left-color: #f3f3f3;
    border-right-color: #f3f3f3;
    border-bottom-color: #bdbdbd;
    border-top-left-radius: $border-radius;
    border-top-right-radius: $border-radius;
    height: 20px;
    text-align: center;
    color: #505151;
    font-size: 12px;
    font-weight: bold;
    line-height: 20px;
    vertical-align: middle;
    background: radial-gradient($btn-radius at 14px 50%, #ff5e57 0px, #ff635a ($btn-radius - 2), #fd5249 ($btn-radius - 1), rgba(255, 255, 255, 0) $btn-radius),
    radial-gradient($btn-radius at 34px 50%, #ffbd2e 0px, #ffc42f ($btn-radius - 2), #fcb91b ($btn-radius - 1), rgba(255, 255, 255, 0) $btn-radius),
    radial-gradient($btn-radius at 54px 50%, #cfcfcf 0px, #d3d3d3 ($btn-radius - 2), #c6c6c6 ($btn-radius - 1), rgba(255, 255, 255, 0) $btn-radius),
    linear-gradient(to top, #cccccc 0%, #d6d6d6 1px, #ebebeb 100%);
    text-shadow: 0 1px 0 rgba(255, 255, 255, 0.7);
  }
  
  &__content-frame {
    display: block;
    background: #cccccc;
    width: 100%;
    height: calc(100% - 20px);
    position: relative;
    
    &:before {
      content: '';
      display: block;
      padding-top: calc(100% - 20px);
    }
  }
  
  &__content {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
  }
}

External CSS

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