function isVisible( row, container ){
    var elementTop = $(row).offset().top,
        elementHeight = $(row).height(),
        containerTop = container.scrollTop(),
        containerHeight = container.height() - 300;
    
    return ((((elementTop - containerTop) + elementHeight) > 0) && ((elementTop - containerTop) < containerHeight));
}

$(window).scroll(function(){
  $('.js-theme-contrast-dark').each(function(){
      if(isVisible($(this), $(window))) {
        $('body').removeClass('theme-light').addClass('theme-dark');
      };
  });
  $('.js-theme-contrast-light').each(function(){
      if(isVisible($(this), $(window))) {
        $('body').removeClass('theme-dark').addClass('theme-light');
      };
  });
});