I’m having the same problem; black bars on the sides of my landing page video using the Twenty Seventeen theme. I can’t figure out how to fix it, here’s what I tried:
– I used HandBrake to reformat my video to 2000 pixels in width. I opted to maintain the aspect ratio which gave me 1126 pixel height. This video plays in the youtube player with full width of the screen.
– I uploaded the video to youtube, then copied the URL that youtube gave me into the spot in the Twenty Seventeen theme where it says: paste youtube URL
– I’m Still getting bars at the sides of my landing page. Note, the width of these bars is the same size as when I use a different youtube video that has 1200 pixel width.