Add example explaining how to use CSS

[sonnyp]

It's confusing in GTK.

The #id selector actually matches the Widget name
object id is for get_object

It can be difficult to figure out how to set css classes via code or builder
maybe list stuff that's not implemented

https://docs.gtk.org/gtk3/css-overview.html
https://docs.gtk.org/gtk4/css-overview.html
https://docs.gtk.org/gtk4/css-properties.html