Seing default CSS? Or default ULX motd?
Pretty sure you copy ulx_motd.txt from addons to gmod_root/ulx_motd.txt, then edit the HTML of that file.
However, its been a long time since Megiddo rewrote the motd code because various other mods kept overwriting the motd.txt, so, I might have it wrong.
You can also edit your /data/ulx/config.txt to include a URL if you want.
See this section of config.txt
ulx showMotd 1 ; Set to 0 to disable showing motd on connect. Shows the file the cvar motdfile says to. Set to a URL to show a URL.
motdfile ulx_motd.txt ; The motd to show