tdf#168240 Sidebar Line Thickness drop down has black background

regression from
  commit 088a7c7c45
  Author: Noel Grandin <noelgrandin@gmail.com>
  Date:   Tue Sep 24 16:18:11 2024 +0200
  remove alpha device from OutputDevice

Change-Id: Id671d55a552e2a0c7011ba516ff1713c7c0d9176
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/192070
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

svx/source/sidebar/line/LineWidthValueSet.cxx

@@ -122,11 +122,6 @@ void  LineWidthValueSet::UserDraw( const UserDrawEvent& rUDEvt )
             pDev->SetFillColor(Color(50,107,197));
             pDev->DrawRect(aBackRect);
         }
-        else
-        {
-            pDev->SetFillColor( COL_TRANSPARENT );
-            pDev->DrawRect(aRect);
-        }
 
         //draw text
         if(nSelItem ==  nItemId )
@@ -160,6 +155,7 @@ void LineWidthValueSet::SetDrawingArea(weld::DrawingArea* pDrawingArea)
     Size aSize(pDrawingArea->get_ref_device().LogicToPixel(Size(80, 12 * 9), MapMode(MapUnit::MapAppFont)));
     pDrawingArea->set_size_request(aSize.Width(), aSize.Height());
     SetOutputSizePixel(aSize);
+    SetColor(Application::GetSettings().GetStyleSettings().GetListBoxWindowBackgroundColor());
 }
 
 } // end of namespace svx::sidebar