Please add class to the shortcode

It is the best way to make the link look like all the buttons in the theme.

Fixed code:

function ninja_forms_modal_form_shortcode( $atts )
{
    extract( shortcode_atts( array(
        'id' => '',
        'class' => '',
        'text_link' => '',
        'image_link' => '',
    // ...etc
    ), $atts ) );
    $link = '';
    if ( $image_link ) {
        $link .= '<a href="#ninja-forms-modal-' . esc_attr( $id ) . '" rel="nf-modal:open" class="nf-modal-link '.$class.'">';
            $link .= '<img src="' . esc_attr( $image_link ) . '" />';
        $link .= '</a>';
    } elseif ( $text_link ) {
        $link .= '<a href="#ninja-forms-modal-' . esc_attr( $id ) . '" rel="nf-modal:open" class="nf-modal-link '.$class.'">';
            $link .= esc_attr( $text_link );
        $link .= '</a>';
    } else {
        return;
    }
...

https://ww.wp.xz.cn/plugins/ninja-forms-modal/