Customizing twenty-twelve theme dimensions

Hi, sorry this is a basic question but I’ve struggled with this to no avail all day yesterday…I’m just trying to resize the center column of this blog so it’s images are centered and expanded to the left and right widget columns:

https://factualfiction.com/lighthouse/

If the space/padding between the columns and the center blog were minimal, only 10-20 pixels, that would also be ideal…

THANK YOU, sorry about the basic question

(Also, are there significant advantages to upgrading this site to a 2017-child theme?)