This commit does the following:
* Install two lightweight themes instead of one
* Introduce a build config to install the themes instead of relying on channel: INSTALL_COMPACT_THEMES
* Change browser-compacttheme to use the new themes instead of the old one
* Remove inferBrightness since the lwt colors are part of the compact theme definision, as opposed
to the devedition theme which could be light or
* Update CSS to use :-moz-lwtheme-darktext and :-moz-lwtheme-brighttext instead of [devtoolstheme]
MozReview-Commit-ID: 4gKU68drlE2
