To edit the background of your website:
- Go to "Edit Site > Site > Customize Your Site Layout"
- Then click button "Create your own custom design"
- Then click button "Select Backgrounds"
- At the "Choose a background" page preview, Click on "Assign Backgrounds to Pages" at the bottom
- Look for the page you want to add a video background to under "Page Title"
- Once you've located your page click on "New Background"
- Select "Video Background"
Then you are done, just click "Return" at the bottom of the page, "Done" on the next then "Return to Design Menu".