Add detection for dark or light theme.

[dristic]

From #2196 (comment)

The windows API does not actually detect dark mode but forces it on. This is the simplest way to detect dark mode without depending on UWP APIs that require windows-rs. If it fails to read the registry it defaults to a light theme.

I tested this locally and the app will use the dark theme if the user's OS is set to dark otherwise it uses a light theme.

It does not detect this if the settings change while running the app but without UWP APIs I believe I would have to poll the registry which does not feel like a great solution.

Thanks!

[vprimachenko]

Consider checking whether a windows event signals the change. WM_THEMECHANGED and WM_SYSCOLORCHANGE come to mind.

Alternatively one can use RegNotifyChangeKeyValue to observe the key instead of polling.

[xStrom]

Thanks! This is much better than always using dark mode and I've confirmed it working and double checked the unsafe code against Microsoft docs.

[xStrom]

@vprimachenko thanks for the specific pointers, I wrote those down in #2190 which tracks progress on this.

This PR here was just a quick follow-up to #2196 and thus limited in scope on purpose.