Landing page created with Gatsby, hooked up with Dato CMS

Hey, I've finished the Easybank landing page challenge. I've created this page using Gatsby (my very first Gatsby project). I added a little feature - hooked the site up with Dato CMS, where I put all the blog posts and created pages for these posts using Gatsby Create Pages. I've got some trouble styling the hero image in the header - I couldn't figure out how to show the overflow on the bottom and hide it on the top (even if I tried overflow-y: visible, hoping that it will hide under the fixed navbar, it turned into a scroll). Any tips on how to fix that?