I think I’ve figured it out… adding the code below to my custom.css file seems to have done the trick:
.page-template-page_blog-php .wrap, .page-template-page_blog-php .site-header {
margin: 0 auto;
max-width: 1140px;
}
.single-post .wrap, .single-post .site-header {
margin: 0 auto;
max-width: 1140px;
}
