Changing content width on posts only

Hi,

I am using the theme Ashe and want to change the content margins on posts only.

Ideally what I want is the pages to stay the same (full width) but for the posts to have:
full width header and footer
full width featured image (if possible, not essential)
85% width for all the content on every post.

I also want this only to be for viewing on a desktop. For phone’s, I would prefer for it all to remain full widths for easier viewing.

I’ve tried adding CSS that I’ve found in other forum posts however I don’t see any changes. I’m not sure if I’m somehow doing it wrong or if it is just the wrong code so if someone could advise me on how to actually add the CSS as well that would be amazing.

At the moment I have just been coping the CSS > going to Appearance > Customise > Additional CSS > Pasting the code > Pressing Publish …and seeing no change

The page I need help with: [log in to see the link]