The header and slide-out menu now match the page's theme, providing a
consistent look on smaller devices. This works for all pages that
provide such a menu.

I've also fixed showing the page mask on the dashboard, fixed the
z-indexes for the mobile menu and mask, and added a transition when
showing/hiding the mask.

Tested the dashboard, review request, and administration pages in a mobile

mode. They all looked as expected (aside from the administration page, which

has some existing visual quirks apparently unrelated to this work).