Can I use HD YouTube vids in WPMU themes?

Hi, I'm trying a WPMU theme for the first time and it seems like the only code that works for the header video is the default, which is the lowest quality so it looks grainy on full screen. I'm trying to insert the HD embed link, but it just shows a blank white screen instead of a video. Is it possible to show anything other than the default?

HD link:
https://www.youtube.com/embed/car1dc70ERI?vq=hd720

Thank you!
Ellie