Get a child theme from https://vamtam.com/child-themes/ and put the following in the child theme's functions.php file:


add_filter( 'body_class', function( $class ) {
    $class[] = 'sticky-header-type-below';

    return $class;
} );


Once you've done that, you can optionally add the following to Customize/Additional CSS:


.sticky-header.sticky-header-type-below .fixed-header-box,
.had-sticky-header.sticky-header-type-below .fixed-header-box {
    top: -40px;
}

.admin-bar .sticky-header.sticky-header-type-below .fixed-header-box,
.admin-bar .had-sticky-header.sticky-header-type-below .fixed-header-box {
    top: calc( 32px - 40px );
}


Both bold numbers should be equal to the height of your top bar.