Child Theme Made Header Too Small

I don’t even know where to find this in the billions of templates and additional CSS, so I’m just asking the forum.

here’s my site: https://www.arthuriansmutcycle.com/

As you can see the header got made bonkers small once I made a child theme to handle displaying posts in ascending/chronological order. Ideally the width is the same as the welcome image beneath it (850 pixels or so), but I don’t even know where to change it myself now that I’m using a child theme. Thanks for any help you can give me!