Not 100% sure what exactly you are trying to do. But the general approach is:
Create a new theme:
cp /usr/share/univention-web/themes/dark.css /usr/share/univention-web/themes/mytheme.css
Adjust the file…
univention-app configure keycloak --set keycloak/theme=mytheme
This is documented here:
https://docs.software-univention.de/manual/5.0/en/central-management-umc/introduction.html#creating-a-custom-theme-adjusting-the-design-of-ucswebs
and here:
https://docs.software-univention.de/keycloak-app/latest/configuration.html#envvar-keycloak-theme
Note that the theme is not supposed to do all kinds of magic. The variables work, but doing anything else (i.e. arbitrary CSS code) may break at some point. At the very least, the structure of the document depends on the Keycloak project.