You can compensate for the padding on pre-content with the rule below in the pre-content BG CSS+ box
{margin-left:-2.7%;width:105.4%;}
The reason for the values are the following.
The content area has a 2% right and left padding. This is computed based on the whole wrapper width.
When applying a margin to the pre-content area, the width reference is the one of the content area which is 74% of the wrapper.
So you need to use -2% / 74% = -2.7% for the left margin
You need to then extend the width with double that (5.4% > 105.4%)
You need to then extend the width with double that (5.4% > 105.4%)