I need to use a dark theme in KDE because I spend so much time on the computer that my eyes get tired of looking at whiteness all the time. I use a very dark theme to minimize the strain on my eyes. If you use LibreOffice and a theme such as Zvon Black, you may have found that your background is totally black and you cannot read documents. Here is the fix:
sudo mv /usr/share/libreoffice/share/config/images_crystal.zip /usr/share/libreoffice/share/config/images_crystal.orig.zip
sudo mv /home/username/Downloads/images_crystal.zip /usr/share/
I took several sets of instructions from several sources, updated them, and created a simplified howto here.