Change height of header

We are using the Book Lite theme on https://www.robpleas.com. If you look at the site, the space for the header is very large. We want to shrink it so that the actual page content isn’t so far down the page. We’ve tried updating the theme files directly and removing the various padding, etc, in the style.css for the header styles. That did not work. Other posts have suggested using a custom CSS plug-in. We tried downloading one and putting in updated css for the header, but that also did not work. Does anyone know how to reduce the size of the header? I see that there is a custom header file with the theme that has a height setting within it, but changing that height setting did not work. I’m not sure how to tell it to use the custom header file. Does anyone know how to do this?