How to decrease space above shaded parbox in Lyx?

I'm using default settings in Lyx for document class etc., and use several small shaded boxes (via Insert -> Box -> Shaded Background). How do I decrease the amount of vertical space above and below these boxes? It is much more than for a Simple Frame box. Thanks!

\vspace{-0.2cm}

• Great, that works. Is there a way to re-define \begin{shaded} or \parbox so that it always includes a \vspace{-0.5cm}% before the normal TeX code?