[Harlowe] Changing background images during play.

Sorry for the newbie question. I did search for and find a similar question somewhere but the answer given assumed a bit more css understanding than I've got. I'm gonna need an idiot's guide.

Basically I want to be able to change the background at certain intervals in the game. I have:

body {
background-image: url("image.jpg");
}

in the stylesheet, but how do I change that dynamically during play?

Thanks in advance.

Stu