LibreOffice was not using my gtk theme (on my devuan netinst, fluxbox, with xfce4-settings-manager, chtheme, and manually written .config/gtk-3.0/settings.ini).
Turns out you simply need to install an additional package:
sudo apt-get install libreoffice-gtk3
Devuan also has a libreoffice-gtk2 package, which I didn’t try.
I haven’t tried a Fedora minimal/net install with Fluxbox and LibreOffice, so I don’t know if the libreoffice-gtk3 rpm is necessary there, but it’s in the repos.
sudo dnf install libreoffice-gtk3
I was impressed by this behavior. Just install an extra .so file, and boom, LibreOffice integrates with your desktop theme. LibreOffice is some great stuff!
Ripped off from How to change themes [closed] – Ask LibreOffice