Theming and backgrounds

I today discovered nextcloud’s manual / advanced theming options through a theme placed in the themes/ directory.
Is there a complete list of theming options available somewhere? Specifically how to define a background for the login screen?

this can be done via web gui.
or to you want script this or do it on the cli?

I want to do it via an advanced theme in the themes/ directory as described here
The reason is: This would allow me to have different brandings for different domains as described here

Do you search something like this?