How to remove space above masthead?

Hello,

I’m trying to add a box-shadow to my site, but it looks weird because there’s space above the masthead. It looks like the space is part of the div “page-wrapper” but I can’t figure out how to get rid of it. I’ve messed with all sorts of padding and margins in the css but nothing worked. I’d like the space between the top and the masthead to be there, I just want to be able to create a shadow that looks like it’s actually around the page. You can see what I’m talking about here (window has to be max size because the theme is responsive).

Thank you!