Using eg Kubuntu with a dark theme may introduce difficulties to see the Libreoffice toolbar icons.
Libreoffice usus the set theme from the desktop environment (DE) and may need to be tweaked to make it more visible.
I use Breeze Dark for the DE and for me, the icons on the Libreoffice toolbars are very dark and next to invisible. This is how to make it better.
More notes, tips and tricks to be added later.
2.1. Toolbar icon sets
# apt install libreoffice-gtk
# apt install libreoffice-style*
# dpkg -l|grep -i libreoffice-style
Then in eg Libreoffice Writer, click menu Tools/Options/View and go through the available icon packs that is optimal for you.