Fixed image size

Site URL: https://andpartners.squarespace.com

Hi, my site andpartners.squarespace.com (password: partner) works with a coded header banner (image). but it has a different height on all browser sizes, so the navigation menu below always has a different height as well, which is very annoying. I want the banner to always be 90% of the screen height and the menu bar 10% of the screen height, no matter the browser size. Is this somehow possible to add through coding? 

Try adding to Design > Custom CSS

body.homepage div#siteWrapper:before {
	height: 90vh !important;
}