Allow the height of the shadow to be changed

[blasten]

Users may want to customize the height of the shadow, so some of these properties can be moved before the @apply.

  #dropShadow {
        @apply(--paper-header-panel-shadow);
        position: absolute;
        top: 0;
        left: 0;
        right: 0;
        height: 6px;
        pointer-events: none;
        -webkit-transition: opacity 0.5s;
        transition: opacity 0.5s;
        opacity: 0;
      }