Changing the header height in Grid market theme

I have a client with a header that he is NOT willing to change. Usually that isn't a problem and I can tweak some CSS and everyone plays nice. That isn't working so I am wondering if it is set in the php and would require a separate child theme instead of just utilizing the custom.css. The required header logo height is 158 instead of the 100. The logo is 980 wide.
Suggestions?

Unfortunately I can't provide a link as the site is password protected. I know that makes it tough