Improve appearance of wxTreeCtrl with wxTR_ROW_LINES in dark mode

Make the lines colour less bright to avoid having too high contrast
between the lines and the background, notably in dark mode under macOS.

See #19116.

src/generic/treectlg.cpp

@@ -2816,10 +2816,7 @@ wxGenericTreeCtrl::PaintLevel(wxGenericTreeItem *item,
 
         if (HasFlag(wxTR_ROW_LINES))
         {
-            // if the background colour is white, choose a
-            // contrasting color for the lines
-            dc.SetPen(*((GetBackgroundColour() == *wxWHITE)
-                         ? wxMEDIUM_GREY_PEN : wxWHITE_PEN));
+            dc.SetPen(wxSystemSettings::GetColour(wxSYS_COLOUR_GRAYTEXT));
             dc.DrawLine(0, y_top, 10000, y_top);
             dc.DrawLine(0, y, 10000, y);
         }