That's not a bug, that's a feature. No, seriously. The background is supposed to be different on the odd and even lines. It helps the reader's eye to scan across the line. This is actually good design practice.
I uploaded ThemeRoller and created my own theme and I like this very much. I have one problem I cannot figure out, though. I'm not sure how to explain it, but in a category or subcategory that has more than one item, every other item the background doesn't appear - it's just white. Here's a web page you can see it...
I'm stopping short of messing with any css. I'm hoping you can help.
Also, what if I don't want the content background in these cells or tables?
If you want to change any of the other backgrounds, header colors, etc., then you'll need to change the CSS class that triggers them. That's in far too many places to list here. I suggest using the Firebug extension in Firefox to examine the element you want to change.