How to modify the grid in Upfront themes

This question applies to all the upfront themes. Under the global background settings I was successfully able to set the contained area to 1200px for desktop, so that contained backgrounds are set to that width. However, for all other container elements, the width is set to the grid (1080px). This makes certain styling difficult -- especially when working with css3 gradients instead of images for the backgrounds.

Is it possible to edit the grid to modify the max width? Or perhaps to set it to percentages?

Thanks!