Fixes https://bugs.winehq.org/show_bug.cgi?id=55640.
The lightening/darkening values are loosely based on Qt 6's fallback code: https://github.com/qt/qtbase/blob/ba98644180bcaf40341a9005abe93575cf45cc5a/s....
The values are not perfect, technically it should use Software\Microsoft\Windows\DWM\AccentColor. But since to my knowledge, wine doesn't use AccentColor in any way, this should do the trick for now. `COLOR_HIGHLIGHT` should be roughly the same color as the accent color in most cases. Qt 6 uses the accent color as its highlight color anyways.
-- v4: windows.ui: Support accent colors in uisettings3_GetColorValue.