Day/night themes

[baka-kaba]

Add the option for an alternative theme, that's applied when the system says it's "night"

AppCompat has some automated functionality for this, but it's tied to extending a theme that automatically goes light or dark, which isn't how our themes work.

Since our theme system has a basic API that takes a forum and internally decides what theme to return, it shouldn't be too hard to add some extra logic. If the system decides to supply the current user-selected theme, it just needs to provide the 'dark theme' if there's one set/enabled in the preferences, and it's currently 'night'. Activities and the WebView both hook into this system, so it should just work once it's returning the correct theme

We just need to be able to get the current day/night status from the system