you mean the 10px margin to the left and right of the rt-grid classes or the 15px-padding of .rt-block?
To see the structure you can click the link in my signature. but I would not change the margin of the grid classes.
I would start with .rt-block and try negative margins to the left and right ... (and take care of the IE7-trouble that might come with that: most of the time zoom: 1; position: relative to the parent element helps)