Add Gradient Nav to Clarkson Template

Site URL: https://pomegranate-clavichord-6l2a.squarespace.com

Would love an assist, please... working on this site:

https://pomegranate-clavichord-6l2a.squarespace.com/

Pass: zilla123

I'd like to use CSS to add a top-to-bottom gradient to the Navigation Bar (currently set to fixed/all-white), but do not know the proper CSS code to make this possible. I've attempted trying a few other working answers here on the forum for other 7.1 templates, but perhaps Clarkson is a bit different from the rest?

Many thanks in advance.

Add to Home > Design > Custom CSS

body[data-menu-overlay-theme-switcher="true"] .header.black {
    background-color: #6119b3;
    background-image: linear-gradient(#6119b3, black);
}