Make the Main Content Box Wider on Mobile View

Hi,

So im looking to widen the main content area on my site. On desktop its fine, but on mobile the main contrent where the text and images are is quite thin, and i would like it to take up the whole width of the mobile screen. I have tried looking all over for options but cant find anything. Im a real beginner with coding as well.

My site is https://www.calumlewis.co.uk if you view one of my journal posts on mobile you will see what I mean