It is unlikely that you will be able to use the CSS for much. You are better off building the custom colors in Lectora.
And since CSS is a browser thing, you will only see its effects when you Preview in Browser or publish to some form of HTML and then fire it up in a browser. Lectora editor uses its own rendering engine. Oh, yes, each browser may give slightly different results (Firefox, IE6, 7, 8, ...). This is a browser issue, not Lectora.
