Stylesheet equivilant of setTextFormat?

Is there a way of changing the styles of parts of a text using stylesheets, but without giving each part its own class? (Like you can do with setTextFormat by supplying the start and end character indexes). Or is there just another way of achieving the same result without using setTextFormat because you can’t use it on a textfield that has a stylesheet applied to it?

I’ve searched for a while now with no results, so any help would be great.