Getting yourself a hitskin theme

First head to hitskin.com, they are the official forumotion theme site. (Note: This site may take awhile to load)
Go to search, and put in the kind of theme you want. I reccomend using your type of forum (wether it's phpbb2 3, punbb or invision) but you can change your type in the administration panel.
Once you find yourself a nice theme copy your forums url into the "Forum Address" box.