[Theme: Benevolence] How to make header img responsive?

I’ve looked around and seen the

.site-header { background-size: 100% 100% !important; }

stuff. That’s not going to help me at all.

Please view the site here: https://www.worldseminary.com/

I would like to know how to make the header logo image responsive while still showing proper size in the browser. Any help much appreciated. Thanks!