@davidfor
Just some minutes ago I solved the problem - see the "edit section" in my post.
But thanks for your offer
Something else:
What do you think about an option to declare a path to a local "kobo_extra.css" instead of using a "kobo_extra.css" on the device?
I think, it's more comfortable for a user, when he could refer to his local "kobo_extra.css" in the configuration of KoboTouchExtended.