Handle undefined theme the right way

Sorry for the confusion.
This is the right way to do #10236.
@MarcelGerber no we haven't ran into issues.  That's because we have this https://github.com/adobe/brackets/blob/master/src/view/ThemeSettings.js#L49.

That discrepancy was introduced in the shuffling when we were trying to land themes.  I have cleaned up lots of that stuff in this PR https://github.com/adobe/brackets/pull/9069 which I think we should really merge, but have not heard much feedback on.