Increase width of main text area of posts

Is it possible to easily change the width of this area. I really love the theme and it looks especially really nice on mobile devices such as phones and I-pads. On a desktop I find the white area a bit too much on both sides. And I don’t want to have to increase my screen viewing option in my browser.

Any great advise will be highly appreciated.

If my understanding is correct current width is 624, if it could be increased to like say 920 it would look nicer on desktops in my opinion.

Site is https://ce-zone.nl/?p=6935

I tried to find the solutions and found this https://www.ads-software.com/support/topic/changing-the-column-width-for-posts-default-theme?replies=4

But don’t know if it will just work if I would copy this to the edit css section of Appearance. I think it needs some editing in the code to properly work.

When adjusting the width, will it have an impact on how the site is viewed on other devices then a desktop?

And when adjusted only the main text area in the single post, will it affect the featured image looks on all different devices.
I probably want something really complicated.

But is there is an easy fix for me, I would be really happy.