Boxed and Stretched Layout

Hello,

I have chosen the boxed-attached layout for my site, but would like to have stretched layout for the mobile version. Is this possible? and if so, what code would I need to insert?

If it is not possible to choose a different layout for the mobile version, is it at least possible to decrease the left and right margins? Please let me know the correct code to achieve this. Thank you!

The attached image is the mobile version where I would like to “get rid of” the left and right margins.