Thank you for reaching out to us. You can add a background video to a section easily using Cornerstone. First add a section then under Setup enable Advanced Background option. You'll see Background Lower Layer and Background Upper Layer options. Under Background Lower Layer select the Video type and input your Video Source in the field (see screenshot)
If you don't see those option then you need to enable the advanced mode first (see screenshot)
Another option to add a background video is using Revolution Slider above / below masthead. To do this edit your page in WordPress page editor, activate the Slider Settings: Above / Below Masthead and then include your video URL in Optional Background Video.
For this you would need to create a transparent slide and slider. To learn more about Revolution Slider please see https://www.themepunch.com/revslider-doc/slider-revolution-documentation/
You can also add a background video using a static layer in Revolution Slider please see https://www.themepunch.com/faq/static-layer-background-video/
Hope this helps!