Add Responsive Video CSS Style to Your Theme

Click on Appearance > Customize on the left main admin navigation bar
Click Additional CSS
Paste the CSS from the grey box in this tutorial below the existing CSS then click publish

Note: Not all themes have the ability for you to add additional CSS. If yours doesn’t offer this feature contact us and we will add the CSS to your theme for you.

 .responsive-video {
      position: relative;
      padding-bottom: 56.25%; /* 16:9 Aspect Ratio */
      height: 0;
      overflow: hidden;
    }
    .responsive-video iframe {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
    }