How do I remove the default styles for a form?

Advanced Forms enqueues three stylesheets when rendering a form — af-form-style (Advanced Forms’ own) plus acf-input and acf-pro-input (ACF’s). Hook into af/form/enqueue and wp_dequeue_style() each handle to drop them.

add_action( 'af/form/enqueue', function ( $form, $args ) {

    // Remove Advanced Forms' default style.
    wp_dequeue_style( 'af-form-style' );

    // Remove ACF's default styles.
    wp_dequeue_style( 'acf-input' );
    wp_dequeue_style( 'acf-pro-input' );

}, 10, 2 );

To scope this to a single form rather than every form on the site, gate on the form key:

add_action( 'af/form/enqueue', function ( $form, $args ) {

    if ( $form['key'] !== 'YOUR_FORM_KEY_HERE' ) {
        return;
    }

    wp_dequeue_style( 'af-form-style' );
    wp_dequeue_style( 'acf-input' );
    wp_dequeue_style( 'acf-pro-input' );

}, 10, 2 );

Tread carefully

Removing these stylesheets is fine when you’re styling the form from scratch and want a clean slate. Without them, though, several ACF field types — the WYSIWYG, date/colour pickers, the media uploader UI, repeater layouts — will render unstyled or partially broken until you provide your own CSS. Dequeue when you’re ready to own the styling end-to-end, not as a quick cleanup pass.

See also

• How to dequeue scripts — same hook, script counterpart.