Changing content area width

I would like to make the content width area of the inkblot theme smaller. Meaning I’d like to make the area where the webcomic / widget area smaller and the background bigger. I’m not sure where or what file to edit.

Going to my site, https://www.lesbionixwebcomic.com , I’d like to make the maroon background area larger and the white content area smaller.

Thanks in advance for your help with this matter!