Hi,

We'll add check/uncheck all buttons in future update.

Meanwhile you can add this code in your theme functions.php file at the bottom:

add_action('wp_footer', function() {
    ?>
    <script type="text/javascript">
        (function ($) {
            if (typeof $.stecExtend === 'undefined') {
                return;
            }
            $.stecExtend(function (m) {
                m.$instance.find('.stec-top-menu-filter-by-category ul li').each(function () {
                    $(this).removeClass('active');
                    $(this).find('i')
                            .removeClass('fas fa-square')
                            .addClass('far fa-square');
                });
                m.calData.categoriesFilter = [];
            }, 'onAddToEventsPool');
        })(jQuery);
    </script>
    <?php
});

This code will uncheck all categories by default.

Place the attached file to stachethemes_event_calendar/front/js/adds folder.