Scoped themes to apply only to the editor (#editor-holder)

https://github.com/adobe/brackets/issues/8415
https://github.com/adobe/brackets/issues/8420
One thing that's worth noting about the prefs change events is that they only signal a *possible* change in the preference value, not a certain one. It may be the case that the flickering you saw was because it was refreshing the editor multiple times when it didn't really need to.