Adds dark class and moves inline-widget color into core UI theme

See [this comment](https://github.com/adobe/brackets/pull/8484#discussion_r15225897) from #8484.

cc @MiguelCastillo to review the ThemeManager change and @larz0 to make sure that the new location for the `.inline-widget` style makes sense.
Yeah, I will probably do that this afternoon as I try to bring in some dark styling to the core UI.