I have tried several things, and libreoffice uses the gtk2 theme when run as root without gdk-pixbuf2.tcz installed, but not as user.
When I build a package natively into /apps/pkgname all seems to work as expected. But using a prebuilt binary can run the risk of some things being expected in certain places, like gdk-pixbuf2. If I had time, I would just build all packages outright into /apps, but I am one person and I don't have the time resources to build every app with deps right now, though most everything works as is and may need a little workaround, like installing gdk-pixbuf2.tcz.
Maybe one day if others are involved in scm package creation, we can build everything from source like with the tcz's and that could eliminate some of these unexpected results.