set the same background colour for wxMDIParentFrame as for wxFrame

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@37389 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/mdi.cpp

@@ -214,6 +214,8 @@ bool wxMDIParentFrame::Create(wxWindow *parent,
       return false;
   }
 
+  SetOwnBackgroundColour(wxSystemSettings::GetColour(wxSYS_COLOUR_APPWORKSPACE));
+
   // unlike (almost?) all other windows, frames are created hidden
   m_isShown = false;