You cannot add a video background but we are working on this. As a workaround, please consider adding an animated .gif file as your background image.
Before you start, you should convert your video file (MP4, WebM, AVI, MPEG, FLV, MOV, 3GP or another format) into a .gif format. There are a number of free tools available which you can find using this Google search or using one of these tools:
You can learn more about how to create an animated background in this tutorial.