You can do this to e.g. have a different background color for the final slide:
Declare a style for the background (given the class "the-end"):
html.the-end .state-background {
background-color: rgba(0,0,0, 0.8);
}
Then have the final slide look like this:
<section data-state='the-end'>
<h1>__END__</h1>
<br>
<h3>Thank you!</h3>
</section>
EDIT (added by tyler rinker) You are correct but I wanted to be more explicit for future searchers. I added the following two segments.
Added this to reveal.min.css
.orange .reveal .state-background{background:rgba(255,165,0,1)}
Added this to reveal.css
.orange .reveal .state-background {
background: rgba( 255, 165, 0, 1 );
}