Fix menu bar colours in dark mode

The colours have somehow got completely mixed up when they were made
customizable in 76fffafcd0 (Allow customizing menu bar menus colours in
MSW dark mode, 2023-02-19) and the menu bar looked completely wrong and
too bright as the result.

Also reuse wxSYS_COLOUR_MENU instead of duplicating its 0x2b2b2b value.

src/msw/darkmode.cpp

@@ -340,13 +340,13 @@ wxColour wxDarkModeSettings::GetMenuColour(wxMenuColour which)
             return wxColour(0xffffff);
 
         case wxMenuColour::StandardBg:
-            return wxColour(0x6d6d6d);
+            return GetColour(wxSYS_COLOUR_MENU);
 
         case wxMenuColour::DisabledFg:
-            return wxColour(0x414141);
+            return wxColour(0x6d6d6d);
 
         case wxMenuColour::HotBg:
-            return wxColour(0x2b2b2b);
+            return wxColour(0x414141);
     }
 
     wxFAIL_MSG( "unreachable" );