Suggested snippets

Needing to enlarge the width of the main content area, I write into Other options of the section Customize menu, the snippet .site-main, .site-footer { max-width: 65em; } without success.
Other snippets i.e. about Change the size of the social icons or Change the color of the social icons, are OK.
I am working on localhost with easyphp 12.1.
Could yo help me? Thanks