add video CSS

[VioletKnight7]

Fix for https://github.com/nanostudio-org/nanogallery2/issues/360

<style>
    .nGY2GThumbnailVideo {
        background-image: url(assets/icons/play-button-1.png);
        background-position: center;
        background-size: 70px;
        background-repeat: no-repeat;
      }
</style>