Depending on your category organization, it can be quite simple to get the "rainbow" effect like that site. You will have to make at least one simple edit to your tpl_categories.php, though, as the stock file uses inline links separated by <br /> tags, and this is not capable of displaying full-width backgrounds.
For starters, add to your stylesheet
Code:
#categories a {display: block; background: #11bbcc;}
#categories a+a {background: #33bbcc;}
#categories a+a+a {background: #55bbcc;}
#categories a+a+a+a {background: #77bbcc;}
#categories a+a+a+a+a {background: #99bbcc;}
#categories a:hover {background: #11bb66;}
#categories a+a:hover {background: #33bb66;}
#categories a+a+a:hover {background: #55bb66;}
#categories a+a+a+a:hover {background: #77bb66;}
#categories a+a+a+a+a:hover {background: #99bb66;}
This will give you different background colors for the first 5 categories, and different colors when hovering over one; extend as required, and adjust colors to taste. You can use background images in this as well.
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