Added support to dark theme (user can force light/dark theme or use the system default)

CR-GUI is a companion app for the great BCR (Basic Call Recorder) Android application. To manage BCR call recordings, you actually need to use a file manager... and thats not so user-friendly 😉. It's hard to "extract" call informations from audio filename, and its even harder to search them when the list grows. Such a feature was already requested, but chenxiaolong (the author of BCR) wants to keep BCR Basic; and I do agree with him! Call recording on Android is a hard work, and I'd like (and hope) him to stay focused on it... recordings management is a job made for BCR-GUI 😂.