Getting a theme icon for UI

[vannilla]

I'm trying to create a simple extension which adds two buttons to the navigation bar, that in a way are similar to the back/forward buttons.
As such, I wanted to use the same icons (maybe with some CSS to change color, if possible), but I don't know how to get them from the current theme.
Can someone give me some pointers? Can it be done at all?
I'd like to make the extension restartless, but if there's no other way I don't mind moving to a classic overlay.

[Lootyhoof]

I wouldn't advise trying to get them from the theme personally, it may be that the specific icon you're looking for either isn't there (see: Linux default theme, which uses stock GTK icons) or is mapped differently to the default theme.

Extensions should generally provide their own icons to avoid any issues.

[vannilla]

I see.
I thought I could pick the icon from some properties of the XUL element or something like that.
I'll make my own icons then.
Thank you.