HC: Avoid same BG/FG colors in flat treeview entry

HC: Avoid same BG/FG colors in flat treeview entry

Selected rows in tree views in HighContrast have a background colour the
same or nearly as the normal text colour, so we cannot let entries in
such rows have transparent backgrounds, or the text inside the entry
becomes nearly or totally impossible to see.

Dodge this by giving entry.flat inside treeview and with :focus the
$base_color, which is different from the text & so lets that be seen.

https://gitlab.gnome.org/GNOME/gtk/merge_requests/125