change menu close button

functions.php

function onepress_site_header(){
        ?>
        <header id="masthead" class="site-header" role="banner">
            <div class="container">
                <div class="site-branding">
                <?php
                onepress_site_logo();
                ?>
                </div>
                <!-- .site-branding -->

                <div class="header-right-wrapper">
                    <a href="#0" id="nav-toggle"><?php _e('Menu', 'onepress'); ?><span></span></a>
                    <nav id="site-navigation" class="main-navigation" role="navigation">
                        <ul class="onepress-menu">
                            <?php wp_nav_menu(array('theme_location' => 'primary', 'container' => '', 'items_wrap' => '%3$s')); ?>
                        </ul>
                    </nav>
                    <!-- #site-navigation -->
                </div>
            </div>
        </header><!-- #masthead -->
        <?php
    }