GMod Source doesn't allow css files using the method ulx_motd.txt file uses.
It's not us, it's a Gmod limitation.
You'd have to use the <style> </style> tag. Use
www.w3schools.org as a good starting place for how.
If you want to host the web pages on your own server, go into XGUI - server motd settings and point to your site.
The external site may still have some trouble with fancy css features though, but is more likely to work over the bare txt method default uses.
