Missing style classes in Granite 7 to port from Gtk3 to Gtk4

I started working on the [Gtk4 port of Mail](https://github.com/elementary/mail/pull/790) and noticed that the following Gtk3 style classes are not available in Granite. Not sure if this is a bug or intentional - in any case, please advise how to migrate them to Gtk4 / Granite 7:

- [`Gtk.STYLE_CLASS_BACKGROUND`](https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_BACKGROUND.html)
- [`Gtk.STYLE_CLASS_SIDEBAR`](https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_SIDEBAR.html)
- [`Gtk.STYLE_CLASS_TOOLBAR`](https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_TOOLBAR.html)
- [`Gtk.STYLE_CLASS_INLINE_TOOLBAR`](https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_INLINE_TOOLBAR.html)