Added SDL_GetSystemTheme() to return whether the system is using a dark or light color theme, and SDL_EVENT_SYSTEM_THEME_CHANGED is sent when this changes

Fixes https://github.com/libsdl-org/SDL/issues/5334
Fixes https://github.com/libsdl-org/SDL/issues/6958
Closes https://github.com/libsdl-org/SDL/pull/6440

src/video/uikit/SDL_uikitviewcontroller.m

@@ -136,6 +136,11 @@ static void SDLCALL SDL_HideHomeIndicatorHintChanged(void *userdata, const char
 #endif
 }
 
+- (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection
+{
+    SDL_SetSystemTheme(UIKit_GetSystemTheme());
+}
+
 - (void)setAnimationCallback:(int)interval
                     callback:(void (*)(void *))callback
                callbackParam:(void *)callbackParam