Dark theme support

[eliroca]

Dark theme support can be added via openSUSE Chameleon design system

Currently, we use a local, slimmed down copy of the Chameleon CSS code, which works well with what MirrorCache renders.
See these places in the code for the current implementation:

• https://github.com/openSUSE/MirrorCache/blob/master/assets/assetpack.def#L152

  ! chameleon.css
  < stylesheets/slim-chameleon.scss
  

• https://github.com/openSUSE/MirrorCache/blob/master/templates/layouts/bootstrap.html.ep#L31

  %= asset 'chameleon.css'
  

Consider using upstream Chameleon which has style definition for dark theme via

<link rel="stylesheet" href="https://static.opensuse.org/chameleon-3.0/dist/css/chameleon.css" />

.. making sure it does not break anything that gets rendered from the templates.