Make generic wxListCtrl lines more visible

Use wxSYS_COLOUR_GRAYTEXT, as wxSYS_COLOUR_3DLIGHT is not visible at all
on the default white background under macOS and is barely visible with
the default GTK theme.

On the flip side, it's too bright in the dark themes, but this is,
arguably, better than not being visible at all.

See #22664.

(cherry picked from commit 7bfad49)

docs/changes.txt

@@ -243,6 +243,7 @@ All (GUI):
 - Fix AUI floating pane position when dragging (Konstantin S. Matveyev, #22533).
 - Add wxStyledTextCtrl support to XRC schema file.
 - Fix script parsing in wxLocaleIdent (Ulrich Telle).
+- Make generic wxListCtrl separator lines more visible in dark themes (#22664).
 
 wxGTK:
 

include/wx/generic/private/listctrl.h

@@ -893,7 +893,7 @@ class wxListMainWindow : public wxWindow
     // get the colour to be used for drawing the rules
     wxColour GetRuleColour() const
     {
-        return wxSystemSettings::GetColour(wxSYS_COLOUR_3DLIGHT);
+        return wxSystemSettings::GetColour(wxSYS_COLOUR_GRAYTEXT);
     }
 
 private: