I have installed LibreOffice 7 on FreeBSD 13.0-RELEASE/Xfce 4.16 and it seems that LibreOffice does not follow the desktop theme, it looks completely different from the rest of applications (all of them are GTK-based, no QT apps). What is missing?
Afaik, Libreoffice binary package is built without Gtk3 support which was considered unstable and having visual glitches.
There is no libreoffice-gtk package like the one you can find in some Linux distros.
I've never tried installing it from source with Gtk3 and Gnome support enabled, but it's probably the only way to fix the color scheme.
LibreOffice built with default options uses Qt so it will follow your Qt theme settings.
You can make Qt follow the GTK theme by installing x11-themes/qt5-style-plugins and setting QT_STYLE_OVERRIDE=gtk2.
It uses the old GTK2 so it doesn't always look great depending on the theme, but it still works.
You can also install whatever Qt theme of your liking and set it the same way.
Or do like me and use misc/qt5ct which is quite handy to configure Qt in a non-Qt environment like Xfce.
I especially like its ability to create custom color schemes. I use the default Fusion theme but with custom colors.