tdf#143673 set an explicit drawing area bg

noticed in gtk darkmode on switching from initial application
to start center

Change-Id: I1cffab97ecd69ca6043531a6b2b5fc34b1ca84f1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/140789
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/sfx2/source/dialog/backingwindow.cxx b/sfx2/source/dialog/backingwindow.cxx
index a213009..e34e02e 100644
--- a/sfx2/source/dialog/backingwindow.cxx
+++ b/sfx2/source/dialog/backingwindow.cxx
@@ -74,6 +74,11 @@ public:
    virtual void SetDrawingArea(weld::DrawingArea* pDrawingArea) override
    {
        weld::CustomWidgetController::SetDrawingArea(pDrawingArea);

        const StyleSettings& rStyleSettings = Application::GetSettings().GetStyleSettings();
        OutputDevice& rDevice = pDrawingArea->get_ref_device();
        rDevice.SetBackground(Wallpaper(rStyleSettings.GetWindowColor()));

        SetPointer(PointerStyle::RefHand);
    }