macosx: Fix views dependent on NSView extension shouldShowDarkAppearance not updating colours correctly

We often update colours for elements in the UI by observing and responding to changes in effectiveAppearance. Unfortunately checking effectiveAppearance after a change notification usually returns the old effectiveAppearance, which means that shouldShowDarkAppearance returns the opposite value we want.

By instead checking the [[NSUserDefaults standardUserDefaults] stringForKey:@"AppleInterfaceStyle"] we can avoid this issue

With fix:

Screen_Recording_2023-01-29_at_19.49.24

Before fix:

Screen_Recording_2023-01-29_at_20.03.19