[1.28] Use colors from system theme not working with the same css as before

[JamesR98]

A lot of mate-themes (such as Green-Submarine) have something like the following:

vte-terminal {
    background-color: white;
    color: green;
}

This seems to no longer work on 1.28 (at least on Fedora).

The following does work:

@define-color text_view_bg white;
@define-color theme_text_color green;

(from here: https://gitlab.gnome.org/GNOME/vte/-/raw/master/src/vtegtk.cc)

Though I'm not sure if defining colors will break any other applications.

[luigifab]

"Sadly", the @theme_text_color is used by qt6-gtk-platformtheme (for example with VirtualBox).

[luigifab]

Strangely, the original version is working when pasted in css tab of GtkInspector.