Use dark theme when run in a Phosh session

Downstreams either don't restyle making us divert from the designs or use squeekboard-restyled adding extra complexity. Make it simple for everybody to use the right theme by default so we have get less incorrectly styled screenshots on the interwebs.

Also contains some enhancements to make squeekboard development more of a breeze which I can also split out if needed.

Closes: #242 (closed)