Cover

Cover is a hide modifier in StepMania.

With this modifier on, depending on what percentage is used, it will either darken or completely blacken the background. This mod is useful for slower performing computers that can't play certain background videos in some files well.

In ITG themes, this modifier is known as "Hide Background" and will disqualify your score from ranking if used. I'm not really sure why. This was fixed in the Simply Love theme.