[Themes] Status bar should be light in the initial themes release

[dangoor]

Since we're not planning to deal with panel styling right now, we're thinking that the status bar should be light in 0.42.

cc @MiguelCastillo @larz0

[larz0]

@dangoor @MiguelCastillo @peterflynn I'm going to switch everything to light UI in #8362 and keep the dark assets in the background, and then we can implement dark UI incrementally.

[dangoor]

Closing as the status bar is light now.