Make Firefox use system assigned icons in Oxygen

Hey guys, as the title states, i'd like to make firefox use system assigned icons (in my case, evolvere) with the oxygen theme. Now, it uses oxygen icons, even though i selected evolvere in the gtk section in systemsettings. It looks like this (the green +):



Even though it was supposed to use nicer monochrome icons from the evolvere set. The icons have been set for root and normal user.

Thank you!