Yes you can't change background music to custom but there is a variety of different musics as shown:
just open settings> GX section > background Music.

@hewnly no hewnly it's not beta just update opera gx if it didn't automatically....
Go to O sing in the top left corner of the browser click it> it will open a menu in the last there is an option of update and recovery...click it and check for updates...sorry I can't send you images as I am on the mobile right noe

Yes you can't change background music to custom but there is a variety of different musics as shown:
just open settings> GX section > background Music.