Extends \newtheorem command. If you say \newshadetheorem{theorem}{Theorem} in the preamble then your regular \begin{theorem} .. \end{theorem} will produce a theorem statement in a shaded box. It supports all the options of \newtheorem, including forms \newshadetheorem{..}[..]{..} and \newshadetheorem{..}{..}[..].

The author is Jim Hefferon. The package is Copyright © 1999 Jim Hefferon.