Improve dark mode detection in wxMSW

Use the registry key corresponding to the selection of "Light" or "Dark"
for the "Colors" in the "Settings" applet, see

https://stackoverflow.com/questions/51334674/how-to-detect-windows-10-light-dark-mode-in-win32-application/

Closes #22020.

src/common/settcmn.cpp

@@ -75,12 +75,17 @@ wxString wxSystemAppearance::GetName() const
     return wxString();
 }
 
+#endif // !__WXOSX__
+
+// These ports implement this function using platform-specific API.
+#if !defined(__WXOSX__) && !defined(__WXMSW__)
+
 bool wxSystemAppearance::IsDark() const
 {
     return IsUsingDarkBackground();
 }
 
-#endif // !__WXOSX__
+#endif // !__WXOSX__ && !__WXMSW__
 
 bool wxSystemAppearance::IsUsingDarkBackground() const
 {