Adjusting sidebar appearance of Twenty Fifteen theme

Hi,

I am wondering if someone might be able to help me adjust the 2015 theme so that the sidebar, instead of looking and functioning like this (i.e. stuck to the left and visually colored by the body:before pseudo element, to instead looking and functioning like this (i.e. a sidebar floating to the left of the main content, and the full content centered in the screen.

Twenty Fifteen layout is almost perfect for me, except for this on layout issue, and I’ve had a really hard time adjusting the CSS myself because of the percentage widths and em widths of various elements really confusing me.

Perhaps the pseudo elements position and width only need to be adjusted slightly to accommodate this change (??) but i haven’t figured it out yet. I think the percentage issue makes it difficult.

Thanks in advance of any help anyone can offer. I will be incredibly thankful for your time and consideration.