That'll move it down, so you can close it from your editor (and once you've done that, you won't need that bit of code anymore, so you can take that out, too).
▧▧▧ ♬ ▧▧▧ In answer to your new question, add background-attachment: fixed; to your body line
As a side note, do you also want to know how to make it fill the whole page? (like this?)
You can use background-size: cover; or background-size: 100% 100%; in order to get that.
i want to autoplay my music on my profile.
but i dont want a media box on my profile either.
can someone explain the code i can use to add to my theme override section?
You need a media panel in order to have media (videos, playlists, etc.) on your profile.
Some people choose to hide it in order to look fancy. While this does look nicer, most people find the lose of control over the music quite annoying (since some people browse the internet with their own music playing and don't want to listen to yours)
Autoplay:
Add ?&autoplay=1 to the end of your youtube url.
If you don't want it absolutely "hidden", there are plenty of other options to stylize a youtube panel into something which looks nicer. For example, my profile has it just as a play/pause button. You can also make it a bar across the bottom like this or make it into a smaller video.