Accidentally, I deleted the /usr/share/themes folder... I reinstalled the light-theme and finally I got my theme back but with one issue: Around some buttons there is a white border: In nautilus https://i.stack.imgur.com/reNMl.png and in gedit: https://i.stack.imgur.com/Xu7jG.png For example the search button on the right side (in the pictures).
If I change the theme to numix or arc, these buttons are completely white... I already refreshed the icon cache, forced unity to reset and tried other themes. Nothing helped.
Thank you for your answer!