判断页面滚动

window.onscroll = function () {
        var t = document.documentElement.scrollTop || document.body.scrollTop;
        if (t > 50) {
          $api.css(
            $api.byId("header"),
            "background-color: rgba(29, 205, 184, 0.5)"
          );
          $api.css(
            $api.byId("nav"),
            "background-color: rgba(29, 205, 184, 0.1)"
          );
        } else {
          $api.css($api.byId("header"), "background-color: transparent");
          $api.css($api.byId("nav"), "background-color: transparent");
        }
      };