Tribox

\usepackage{pstricks, pstcol, pst-grad}

\pstribox[shadow=true, fillstyle=gradient,
          gradbegin=green, gradend=red]
  {\color{white}$\left(\begin{array}{cc}
    \cos\varphi & -\sin\varphi \\
    \sin\varphi &  \cos\varphi
    \end{array}\right)$
  }