Black bars?

I really like the simplicity and the minimalistic look of this theme. I just need to make it looking more boxed – having like 2 vertical bars on each side, when looking at it on a PC screen – when using Smartphones and tablets it looks great already, but there is to much white at the sides when looking at it on a pc monitor. I have messed around with margins and padding, but it does not quite work. So how could this be done easily while maintaining the look on smartphones/tablets?