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.
To be future-proof, we should do it like this:
``` SCSS
.dark {
  .inline-widget {
    color: #333;
  }
}
```