Don’t know why but while traversing through the DOM I noticed the CSS link showed:
<link href="http://assets/...
so I filled the seemingly missing domain name in the config box.
That actually doubled the domain making the href read:
http://www.domain.com/www.domain.com/assets/...
No use either.
Then I tried to leave out the domain and the prefix slash in the config box making it read (opposed to the accompanying suggestion)
assets/templates/<etcetera>... (notice the missing slash at the beginning?!)
And, tadaa, my styles were back!
Hope it works for you as well.