These changes are mostly aesthetic in nature. I just thought it would
be a good idea to have theme loading be parallel to package loading as
much as possible. So I localized more logic on the `atom` global.
We'll look first in the user themes directory, then in the built-in
themes directory. This allows us to default to 'IR_Black' and not crash
if the user doesn't setup their `~/.atom/themes` directory.
