Different background of level1 slides in revealjs

[january3]

I would like to have a different styles for some slides, notably the level 1 slides (by default these with h1). Basically, these slides should
be light-on-dark, and all others dark-on-light.

I tried the following:

# Part II {.separator}

And defining the .separator class with

.separator {
  background: black;
  color: white;
}

However, this changes only the background for the tight box around the actual contents, not of the whole slide.