Graphite theme updates to add green variant

This commit factors out the main CSS file for the Graphite theme into
`base.scss` which must be compiled using SASS. There are now two
variants, `graphite.css` contains the original grey colour scheme and
`poker.css` contains a dark green background scheme. Neither of these
should be changed directly but instead generated from `base.scss` - they
are included for users who don't have the SASS compiler installed.