Enable icon themes for Windows

Windows 10 "dark mode" is not supported and likely won't be,
but high-contrast mode and manual color theme editing is still
possible.

common/advanced_config.cpp

@@ -345,7 +345,7 @@ void ADVANCED_CFG::loadSettings( wxConfigBase& aCfg )
     configParams.push_back( new PARAM_CFG_BOOL( true, AC_KEYS::DrawBoundingBoxes,
                                                 &m_DrawBoundingBoxes, false ) );
 
-#ifdef __WXGTK__
+#if defined( __WXGTK__ ) || defined( __WXMSW__ )
     bool defaultDarkMode = true;
 #else
     bool defaultDarkMode = false;

libs/kiplatform/msw/ui.cpp

@@ -53,7 +53,14 @@ bool KIPLATFORM::UI::IsDarkTheme()
 
     return ( val == 0 );
 #else
-    return false;
+    wxColour bg = wxSystemSettings::GetColour( wxSYS_COLOUR_WINDOW );
+
+    // Weighted W3C formula
+    double brightness = ( bg.Red() / 255.0 ) * 0.299 +
+        ( bg.Green() / 255.0 ) * 0.587 +
+        ( bg.Blue() / 255.0 ) * 0.117;
+
+    return brightness < 0.5;
 #endif
 }