To answer you question, Apollo is licensed under the Apache License.
That being said, I would prefer you stick to side loading because I'm
working on releasing it into the the Play Store and plan to do so very
soon. I appreciate you asking first, some people don't and it's
frustrating to see them given credit for my work.
About what you're doing, have you considered using CM's theme engine
to create your themes? Considering you want to give every app the holo
dark theme, that's what I would look into. Also, you should look into
using Apollo's theme engine. If you created a holo dark theme for
Apollo you would also be helping me and Apollo out by providing more
themes for people. Also, since I've released it for CM, I've changed
the default theme to more of a holo dark one. I haven't pushed these
changes yet, but I'm trying to get everything ready to do so very