All my buttons in Safari, Mail, Finder a.s.o. are empty, the icons are missing.

All of a sudden all buttons in Safari, Mail, Finder and most other applications are empty. There are no icons visible (see screenshot)

Also, the performance of my macbook pro has decreased dramatically.

I have scanned for a virus with no result. I did find a four plug ins for safari that I never installed myself, I have removed them.

I don't know what's wrong here or what to do next?

Thanks!

Niels