Change CSS rule for small screen with jQuery
Resolved. 

if ($(this).hasClass('is-active')) {


doesn't allow to show menu links when no class is attached:

else {
          $('ul.menu').css('display', 'none');
        }


So, I modified checking for small screen:

 $(window).resize(function(){     
        if ($(window).width() &gt; 720 ){
          $('ul.menu').css('display', 'none');
        } else {
          $(this).toggleClass('is-active');
        }
      });