I've been using the following in Win 8.1:

  /* to hide the native top tabs */
  #TabsToolbar * {
      visibility: collapse;
  }
  /* to give the top toolbar a bit of height so the window controls don't dangle off to the right */
  #TabsToolbar:before {
    content: '';
    display: block;
    width: 100%;
    height: 20px;
  }

  /* to hide the sidebar header */
  #sidebar-header {
      visibility: collapse;
  }