Genesis filter classes in content area.

functions.php

<?php
//* do not include php tag
 
add_filter( 'genesis_attr_content', 'custom_add_content_attr' );

function custom_add_content_attr( $attributes ){
        
        // add itemscope
        $attributes['class'] = 'content first two-thirds';
        
        // return the attributes
        return $attributes;
        
}