controlling .post widths

I’m still having trouble controlling the widths of customized page elements. I looked at the css code for Decode and tried to follow the same settings. I wanted the page width for my ‘white page’ customisation to follow the width of the .site-header, but though I have used the same settings and applied them to .author and .post, I have not been able to control .post. Author seems to be at least have a max-width fixed, but not .post, either on the blog page or the individual post page.

This is what I have in my custom css:
@media only screen and (min-width:68.5em) {.author, .post {max-width:70em}}

@media only screen and (min-width:50em){.article., site-main., .post {width:80%}}

site is say-write.com (language services, testimonials and home are pages, and blog are posts.