Every application has a dark mode now or will surely have a dark theme in the near future.
I am running openSUSE Tumbleweed with KDE Plasma and Materia Manjaro Dark theme. It's almost like the Breeze Dark theme of openSUSE/KDE but with slightly more greenish elements. Whether using dark themes in GNOME or KDE I am usually left with a not so pleasing toolbar in LibreOffice.
Until today, I didn't bother much about it. Then, I thought of changing that and to my surprise it was just a few clicks way.
Tools > Options
Set the icon style to "Breeze (dark)" and it's done.
Et voilà! :)