Appearance -> Customize -> Theme Options -> Layout
many options are available, default and storefront. To enable storefront layout, install WooCommerce.
You can change header layout for each page from editor page settings as below.
Header style tab has 3 style. Default is header line. Box Shadow and none are other options. If you have a full with slider or featured content with background image, select header style as none. It will remove borders and shadows above the content.