modify_footer.php

<?php // <- you probably don't need this tag
function dt_remove_default_footer() {
    remove_action( 'dt_footer_copyright', 'dt_footer_copyright_default', 10 );
    add_action( 'dt_footer_copyright', 'custom_dt_footer_copyright_text', 10 );
}
add_action('init', 'dt_remove_default_footer');

function custom_dt_footer_copyright_text(){
    echo 'My copyright text';
}