[Theme: twentyfifteen] Sidebar (header) width

Hey all,

I’m looking to make the sidebar on my site narrower. Here’s my site: https://www.thatswhatcheezsaid.com

Here’s a topic I found, but I’d like some explanation of the screenshots in this topic: https://www.ads-software.com/support/topic/reducing-width-of-sidebar-of-twenty-fiteen-theme?replies=21.

I found the body:before area of my css sheet, but the changes that they made in the above thread did not work for me.

Any ideas?