Well, it's definitely happening in the <head> of your source. I checked out our site, and we don't have any inline styling like that when viewing source (also using an Artisteer skin), so I don't think it's coming out of mojoPortal. I would start by looking at your skin's layout.master file and see if it's embedded in there.
Inspecting your site with FireBug, I also see that your head has an ID of "Head1". Ours is "ctl01_Head1". I know mojoPortal uses at least some of those IDs, but I don't know if this particular one might be a problem.
I hope that helps out.