[Mac] Popover appearance on High Sierra and older
[Mac] Popover appearance on High Sierra and older

NSAppearanceNameDarkAqua is not available before Mojave.

This also fixes the build on systems with an older Xamarin.Mac
version (incl. our own CI)

mono/xwt,lytico/xwt,antmicro/xwt,hwthomas/xwt