Change post image width

As one of the previous topics requested (https://wordpress.org/support/topic/is-there-a-way-to-reduce-the-size-of-the-featured-photo-on-post-pages/) … I would like to do this as well, however the background behind the post title, category and date doesn’t change width with the code provided in that previous topic.

The page I need help with: [log in to see the link]