Add extra class to HTML elements in Genesis. Source: https://www.wpstud.io/add-extra-class-to-html-elements-in-genesis/

add-extra-class.php

<?php

//* Add extra class to an HTML element in Genesis
add_filter( 'genesis_attr_content', 'customprefix_add_class' );
function customprefix_add_class( $attributes ) {
    $attributes['class'] = $attributes['class']. 'your-extra-class'; // change "your-extra-class" name
        return $attributes;
}

genesis-filters-list.md

Existing Class Names	Genesis Filters to Use
body	genesis_attr_body
site-header	genesis_attr_site-header
site-title	genesis_attr_site-title
site-description	genesis_attr_site-description
header-widget-area	genesis_attr_header-widget-area
nav-primary	genesis_attr_nav-primary
nav-secondary	genesis_attr_nav-secondary
nav-header	genesis_attr_nav-header
structural-wrap	genesis_attr_structural-wrap
content	genesis_attr_content
entry	genesis_attr_entry
entry-image	genesis_attr_entry-image
entry-image-widget	genesis_attr_entry-image-widget
entry-image-grid-loop	genesis_attr_entry-image-grid-loop
entry-author	genesis_attr_entry-author
entry-author-link	genesis_attr_entry-author-link
entry-author-name	genesis_attr_entry-author-name
entry-time	genesis_attr_entry-time
entry-modified-time	genesis_attr_entry-modified-time
entry-title	genesis_attr_entry-title
entry-content	genesis_attr_entry-content
entry-meta-before-content	genesis_attr_entry-meta-before-content
entry-meta-after-content	genesis_attr_entry-meta-after-content
archive-pagination	genesis_attr_archive-pagination
entry-pagination	genesis_attr_entry-pagination
adjacent-entry-pagination	genesis_attr_adjacent-entry-pagination
comments-pagination	genesis_attr_comments-pagination
entry-comments	genesis_attr_entry-comments
comment	genesis_attr_comment
comment-author	genesis_attr_comment-author
comment-author-link	genesis_attr_comment-author-link
comment-time	genesis_attr_comment-time
comment-time-link	genesis_attr_comment-time-link
comment-content	genesis_attr_comment-content
author-box	genesis_attr_author-box
sidebar_primary	genesis_attributes_sidebar_primary
sidebar-secondary	genesis_attr_sidebar-secondary
site-footer	genesis_attr_site-footer