https://lazka.github.io/pgi-docs/Gtk-4.0/classes/CssProvider.html
Gtk.CssProvider - Classes - Gtk 4.0
gtkclasses
https://mail.gnome.org/archives/commits-list/2012-September/msg08230.html
[gtk+] cssprovider: Add parsing support for @keyframes
support forgtkaddparsingkeyframes