There are a few things that you should look at. In the CSS setting your defaults is only half the battle. You also need to make sure that you don't have any specifics set. I'm not sure what layout you baselined on, or if you started from scratch. How many panes are you using in your page? Did you set the spacing to zero in the ASCX file as well. I find that the ASCX controls the spacing between different table cells, and then the CSS controls spacing inside a cell.
Also, look at your layout. Make sure that all of your colspan='s are set correctly. One of my favorite ways to debug a skin is to change the colors on a 'live' site (localhost is great). Once you figure out that it's the way you want it, replicate the changes up to the real site. If you can give me more specifics, or a copy of your skin, I may be able to help you out more.