Dymok

$(window).scroll(function() {
    var top = $(document).scrollTop();
    if (top < 300) $(".top_nav").removeClass('fixed');
    else $(".top_nav").addClass('fixed');
});

.fixed{
  position: fixed;
}

@media screen and (max-width: 575px) {
  .grid-item {
    left: 50% !important;
    transform: translateX(-50%);
  }
}

Keypress            Command
Ctrl + K, Ctrl + U  Transform to Uppercase
Ctrl + K, Ctrl + L  Transform to Lowercase