Feature Request: How to use CSS for styling?

[lquenti]

Hi! First of all: Thank you so much for your work, I would probably already have given up if I had to only rely on the official GTK documentation. One thing that I'd still love to have:

When the application has to fit a more specific style, CSS seems very useful. But there is just very little information on how to actually use it! I found this one the most useful.... Although I still have to figure out how to actually use it, I think this would be such a great addition.