Hi!
Using GtkBuilder XML, you can currently only add Styling Information like so:
<object class="Foo">
<style>
<class name="predefined-class" />
</style>
</object>
So (please correct me if I'm wrong) CSS cannot be added without actions outside of this XML File. You always have to create a GtkCssProvider and add it to the widget's GtkStyleContext in your programming code.
Wouldn't it be more consistent to allow something like this:
<object class="Foo">
<style>
<provider path="path-to.css" />
<class name="mycustomclass" />
</style>
</object>
This would separate the GUI and the actual programming Logic way better and thus make building GUIs with tools like Glade easier.
Cheers,
Lukas