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

https://github.com/adobe/brackets/issues/8415
https://github.com/adobe/brackets/issues/8420
Does this need to guard against `metadata.theme.file` being undefined? I guess the file just wouldn't exist, which is not such a big deal.

Also, the two built-in themes need their metadata updated. (I've done this in my local copy... if the changes required are minor enough, I will likely just make them and then merge so that I can work from this when updating the themes...)