Ok. Don't copy & paste that, cause I just typed it free-hand & there are probably some issues with the way I wrote it, but that's the basic premise. I would put in the vast majority of the URL itself into the template header. Then use really simple terms for the token "Background". Like - finance, travel, healthcare
That should work just fine.
If not you can definitely add a token to use an external css file. Put the entire CSS url into a plain text token & it will be fine.