Fix scrollbar UI issue on .tree-view

[jsveron23]

When I was using 'spacegrey-dark-ui' theme and expand folders until scrollbar appear on .tree-view, then default Webkit scrollbar appears instead styled scrollbar. This pull-request is resolving this issue and also, styled scrollbar will come up on .tree-view.

Since @cannikin doesn't use Atom editor anymore, you can resolve by following this code on your stylesheet,

.scrollbars-visible-always {
  ::-webkit-scrollbar,
  /deep/ ::-webkit-scrollbar {
    width: 1em;
    height: 1em;
  }

  ::-webkit-scrollbar-corner,
  /deep/ ::-webkit-scrollbar-corner {
    background-color: #1C1F25;
  }

  ::-webkit-scrollbar-track,
  /deep/ ::-webkit-scrollbar-track {
    background-color: #2B303B;
  }

  ::-webkit-scrollbar-thumb,
  /deep/ ::-webkit-scrollbar-thumb {
    background-color: #232830;
  }
}

I hope this commits is fixing #27 issue.