One Column for Posts Page

In a child theme, add

single-post #secondary {
display: none !important;
}
.single-post #primary {
width: 100% !important;
}

And make adjustments from there.

So far, the biggest problem I’ve found is getting WordPress to size ‘Large’ images to my specifications, and Jetpack galleries to follow the content width. Both of those follow some functions in the Twenty Seventeen theme that I haven’t found a way to kill yet.

Thanks for the help.

This is not quite what I was looking for, though. I was hoping to change the page that displays the list of blog posts, not the single posts themselves.

This is what I used to get one column on my main blog/summary/excerpt page. I removed the sidebar widgets and changed the index.php file to show excerpts.

body:not(.twentyseventeen-front-page) .entry-header {
text-align: center;
}
.single-featured-image-header {
max-width: 94%;
margin: auto;
}
.blog:not(.has-sidebar) #primary article {
width: 94%;
margin: auto;
float: none;

}