Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

const
   WM_THEMECHANGED = $031A;

 type
   TForm1 = class(TForm)
     {...}
   private
   public
     procedure WMTHEMECHANGED(var Msg: TMessage); message WM_THEMECHANGED;
   end;

 {...}

 implementation

 {...}

 procedure TForm1.WMTHEMECHANGED(var Msg: TMessage);
 begin
   Label1.Caption := 'Theme changed';
   Msg.Result := 0;
 end;