[epidemian]

I couldn't think of a very good example, but here's something: http://codepen.io/anon/pen/vuyAI

The CSS use a couple of custom properties that define the foreground and background colors. Then, some elements redefine those custom properties (through CSS or JS) and that in turn makes all their children inherit the new custom properties' values and change their styles. This makes the rules that use the custom properties very general, and avoids the need to write more specific rules for the alternative colors. I hope this makes sense :)

I don't think this is possible to do with CSS preprocessors.