Source html, not ULX, won't allow other files (css/js/jpg/gif/whatever) to be referenced on the local file system from a local html file unless it's hosted on a publicly accessible http server somewhere.
ie, <style=css href="
http://mysite.com/mystyle.css">, but if you're going that far, might as well use your own web server link as Avoid suggest.
If only using the local txt file, it would need to be done inside the original file. HTML/CSS is fine, within limits of what the Source HTML/Gmod allows for. (I remember having issues with dynamic div tags/dropdown years ago, it might be better now)