Post customization

Hi, I’d like to ask you one question about the possibility to create a ‘custom style’ post.
I’d want my posts to be centered (like in the template without sidebars) but narrower, let’s say 700px and I’d want to set the fonts of my posts (h1, h2, etc…) in a different way from the standard (but only in the posts, not in the website). I created my first post today but I haven’t found any configuration for doing that, is there a way that I haven’t found?

Thank for your help.

Ticket