Correctly honour theme foreground colour

Rather than try to explicitly look up the colour, don't specify a colour at all. Argument names are never nested within a applying a different foreground colour.

This does have one visible effect: with Adwaita (light), selecting the treeview row causes method argument names to switch from black to white, just like the method name. I think this is actually more readable (and the green used for the signature is illegible against the blue selected-row highlight.)

As discussed in https://gitlab.gnome.org/GNOME/d-feet/merge_requests/4#note_319094 and !8 (closed).