How do I apply style?

Still having problems with this one. When I select and highlight a range of text, I go to style menu in the ruler and select a different style - for example Main Body. Nothing happens. What should I do to apply an existing style to selected text?

Have you tried something other than Main Body? It might be for some reason Main Body got reset to whatever it is you are already using, and is applying it, only of course there is no indication that anything has changed since you are applying A to A.