Startup Pro: Header Media
The Hero Section background image or video can be changed from Appearance → Customize → Header Media. You have the choice of uploading an image or a video, either by uploading a video to your site or by using a YouTube URL (recommended). The recommended dimensions for header images are 1280 x 720, an aspect ratio of 16:9 although this is not required.
If you have chosen to use a video, the header image will be used as a fallback while the video loads. It will also be used as the hero image on inner pages, but will be overridden by the page or post's Featured Image if one is set.
Please note that the header video will not be shown on mobile devices. Most small devices do not allow autoplay therefor the video will not work. Instead the Header Image will be shown.The Header Media Opacity setting shown below will need to be increased for the background media to become visible: