Resize margin for only top post?

Hello. I want to resize the top margin but for only the most recent post on my page. If I resize the top margins for posts, it changes all of them and then they are very far apart. I only want to make space between my header and the most recent post. If I edit the header’s margin, it will only show in portrait mode if I go on my iPad. I want space between the two in landscape. My website is davidboglin.com and I’m using the twenty fifteen theme.

Thank you.