button.js

$(document).ready(function() {
  $(".dropdown-content a").click(function() {
    $(".background-mask").show();
    $(".demo-card-wide.mdl-card").show();

    $(".mdl-card__supporting-text").text(
      $(this).attr("data-content")
    );
  });

  $(".mdl-button").click(function() {
    $(".demo-card-wide.mdl-card").hide();
    $(".background-mask").hide();
  });

});