Modify CSS through add-on

The following example describes how you can modify the browser CSS through an add-on. But where can I find all the CSS the browser uses so I can customize things further?

I had to look for it too as it was new to me.

Here are the list of the available options:

@erosman Thank you. So it seems like you can’t add completely custom CSS for a theme. Like you can for the file userChrome.css

It would be very nice if there was an option to change CSS for the browser itself. I wanted to change a bit how the tabs are displayed. It was partially described here but when applying the changes written down there you get some display bugs. So there is more CSS that needs to be changed. I just don’t know what that CSS.