Thanks for the kinds words! I had made the assumptions that either the header image would be large (1600px x 200px) and should get cropped as necessary at narrower browser widths… or the image would be a small pattern that gets repeated to fill the full browser width (up to 1600px, though, as that is the max width of the entire theme). So that’s how its coded.
In looking into this further, I like the way the WordPress TwentyThirteen theme handles custom header images. They are responsive and it looks like it has a breakpoint at 768px that also adjusts the text size and height.
I’ll look into changing how this theme works. Originally I was worried about changing how headers work since there are over 20K downloads already, but I suspect that lots of folks would want it to work this way.
I wish I had a quick fix for you. You might be able to pull this off with some CSS, but in supporting the WordPress Customize capabilities, some of the header styling is added as inline CSS by the header.php and /inc/customer-header.php files.
I’ll marked this as closed, but am happy to keep the discussion going on it. Let me know if you have more thoughts on this and/or come up with some CSS to change the behavior.