Within the timeline drawing code, adding a class to the Gtk::StyleContext on the fly seemingly did not work.
Now we're doing basically the same here in this small Gtk test application,
and it does not work either :-)
- add a separate dummy Gtk::Frame widget
- apply custom styling to that frame, by virtue of a CSS class '.experiment'
- pick up the Gtk::StyleContext of that testFrame
- use this style context to draw a custom frame onto the canvas
- control extension of that custom frame through the top margin of testFrame
...to find out about GTK's implementation of some aspects of CSS
through Gtk::StyleContext and friends
Basically this is a clone of the existing gtk-canvas-experiment application
