The text inside of ulx_motd.txt is actually HTML.
As for CSS, or any HTML, it all depends on what the actual Source html interpreter allows, not Gmod itself.
Unless it's been improved in the past 2 years, Source html interpreter is very limited as to what it allows.
I've used CSS within the motd html before, but I put it inline using the style tag, not a separate file.
You may use a separate CSS file, but only if you host it on a publicly accessable web server and use the <link> tag to link to it.
Attempting to use locally stored images/files within the Source html won't work.