lubuntu – How can I change Okular’s GUI so I get white text on black background or similar, instead of blending colors?

You need to change the theme for Qt apps.

This can be achieved in two ways, with qt5ct and kvantum (don’t use both of them together).

Kvantum

Install Kvantum with

sudo apt install qt5-style-kvantum

Now make KVantum as the default theme manager for Qt apps.

echo "export QT_STYLE_OVERRIDE=kvantum" >> ~/.profile

Reboot.

Now open Kvantum Manager, and apply a suitable theme.

Install qt5ct with

sudo apt install qt5ct

In Ubuntu 22.04, you will also have to install qt5-style-plugins

sudo apt install qt5-style-plugins

Now make qt5ct the default theme manager for Qt apps.

echo "export QT_QPA_PLATFORMTHEME="qt5ct"" >> ~/.profile

Reboot.

Now open qt5ct and choose style gtk2.