Fixed Site Navigation with solid white bar background: York Templates

Site URL: https://jasper-demo.squarespace.com/

I'm using the Jasper demo/ York template. Just curious if there is a work around to having the site header be fixed and have the whole header with a solid white bar background, while maintaining the left, right, top and bottom padding. Thanks in advance.

Add to Design > Custom CSS

header#header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    z-index: 999;
background: #fff;
}
div#content {
    margin-top: 8vw;
}