theme_text_color updated to theme_fg_color

There is no theme_text_color in Gtk 3.14, this
results  in dark-on-dark unreadable widgets using
Adwaita dark variant.

text_color is equal to fg_color in Gtk 3.6, so the
dependency has been increased.

See: https://bugzilla.gnome.org/show_bug.cgi?id=737340