Actually it is not an unwanted gradient. There is suppose to be a gradient there when you mark a cat as not having thumbs. I always turn off the show post counts for categories without thumbnails in the admin options so there is a full line gradient bar which is desirable.
In fact when I view your sight you have somehow altered the css to take out that gradient of the phpbb plus does not have a catHead class
<td width="45%" class="catHead" align="left">
<table width="100%" border="0">
<tr> <td class="catHead" width="15">
<a href="http://bayareakiteboarding.com/review/index.php?cat=2">Kites</a></span></b><br /><span class="gensmall">Reviews of kites - LEI, Foil, Trainer, etc.</span>
That area is white to me and should have the gradient so you might want to undo whatever you did.