I know that custom colors schemes can be added to a custom theme (which would let admins select between multiple color schemes) - can I add other custom properties? For example, I'd like to add the ability to type in a name (a dropdown would be even better, but that's not critical) to switch between custom headers between spaces. If this is possible, how do I reference them in my theme? I'd love to be able to look this stuff up, but there's REALLY sparse documentation on building custom themes.