FR: Support detecting dark mode in other operating systems

Presently this is only a feature for macOS via juce::Desktop::isOSXDarkModeActive().

The other operating systems provide some form of detection:

I feel like extending this FR to include detecting when a change from light<->dark mode happens.

Also, I just found a Win32 way of detecting such a change, based on this StackOverflow post

String comparisons aren’t ideal of course.