Twentyfourteen-decrease left padding of main content area

Hello!
I am using twentyfourteen to set up a heavily text-based site.
For this site, I find it’s better to leave more space for type –
I would like to shrink that wide left margin (let the type come closer to the left navigational sidebar).
How can I decrease the width of that space between the left sidebar and the main content area?

(And yes, I am working on a Child Theme!)

Thanks so much for any guidance.