Code goes to your active theme’s functions.php file:

function wpcc_wpcfe_print_html_head_cb(){
    ?>
    <style>
        * {
            font-family: DeJavu Sans !important;
        }
    </style>
    <?php
}
add_action('wpcfe_print_html_head', 'wpcc_wpcfe_print_html_head_cb');