A dark theme can be quickly set for the book, but then the table of contents and bookmarks are still very light. That can be very annoying and it would be good if that could be changed.
I have already asked in a Linux forum. There I was recommended the commands:
sudo apt install qt5ct qt5-style-plugins
export QT_QPA_PLATFORMTHEME=gtk2
export QT_QPA_PLATFORMTHEME=qt5ct
But hasn't changed anything. They said someone here would know how to do it
I use Xubuntu. If more information is missing, please ask. Thanks for the help.