LibreOffice 5.4 not using GTK3

I am on Ubuntu 16.04 and using LibreOffice

The program doesn’t use GTK3, no matter what I try. I tried setting the SAL_USE_VCLPLUGIN-variable to gtk3 and I even reinstalled the whole package. However, nothing works.

Does anybody know how to solve this?

Not sure about ubuntu, but on openSUSE GTK3 integration is installed as a separate package. Check on your package manager for something like libreoffice-gtk3 and install it.