framework/less/common/mixins/header-background.less

.header-background() {
  background: @header-bg;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: @zindex-header;
  border-bottom: 1px solid @control-bg;
  .transition(~"box-shadow 0.2s, -webkit-transform 0.2s");

  @media @phone {
    height: @header-height-phone;
  }
  @media @tablet-up {
    height: @header-height;
  }
}