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
 {

src/msw/settings.cpp

@@ -32,6 +32,7 @@
 #include "wx/msw/private.h"
 #include "wx/msw/missing.h" // for SM_CXCURSOR, SM_CYCURSOR, SM_TABLETPC
 #include "wx/msw/private/metrics.h"
+#include "wx/msw/registry.h"
 
 #include "wx/fontutil.h"
 #include "wx/fontenum.h"
@@ -359,3 +360,24 @@ extern wxFont wxGetCCDefaultFont()
 }
 
 #endif // wxUSE_LISTCTRL || wxUSE_TREECTRL
+
+// There is no official API for determining whether dark mode is being used,
+// but // HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize has
+// a value AppsUseLightTheme = 0 for dark mode and 1 for normal mode, so use it
+// and fall back to the generic algorithm in IsUsingDarkBackground() if it's
+// absent.
+//
+// Adapted from https://stackoverflow.com/a/51336913/15275 ("How to detect
+// Windows 10 light/dark mode in Win32 application?").
+bool wxSystemAppearance::IsDark() const
+{
+    wxRegKey rk(wxRegKey::HKCU, "Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize");
+    if ( rk.Exists() && rk.HasValue("AppsUseLightTheme") )
+    {
+        long value = -1;
+        if ( rk.QueryValue("AppsUseLightTheme", &value) )
+            return value <= 0;
+    }
+
+    return IsUsingDarkBackground();
+}