How to get rid of this white line under header image?

I redid a website in GP using hooks to set up 2 header images, one for desktop and one for mobile. There's a white line underneath the header image in both, and I have tried everything to get rid of it -- I've used the Inspect feature in both Chrome and Firefox, and I still can't figure out what's generating it and how I need to tweak the CSS to get rid of it. I want the nav bar to be right underneath the header image without the white line. Can you help me before I lose my mind?!