Restrict content width on large screens on single pages, no sidebar??

I have tried to add css code to the customizer;

@media screen and (min-width: 1200px) {.no-sidebar #primary, .no-sidebar {margin-left: 15%; max-width: 1200px;}}

but all the code I put in the customizer is being ignored?

I need the site to be fullwidth, not boxed because of the front (posts) page. I just want the actual content on all the single pages with no sidebar to be limited to 1200px (and centered) on large screens.

It’s probably really simple, but I can’t see the answer right now.

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