At least in GNOME, LibreOffice honors the GNOME theme selected via
GNOME Tweaks. Launch
gnome-tweaks and select a dark theme in
Appearance>
Themes>
Applications (provided you have any such theme installed). I've just tried it with
Adapta-Nokto,
Adwaita-dark and
Arc-Dark: they all work as expected.
Obviously, this change would render all conform applications in dark tones, not only LibreOffice. This may not be what you want.