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\documentclass{scrbook}

\usepackage{amsthm}
\usepackage{shadethm}

\newshadetheorem{definitions}{Definition}

\newenvironment{definition}[1][]{%
  \definecolor{shadethmcolor}{rgb}{.9,.9,.95}%
  \definecolor{shaderulecolor}{rgb}{0.0,0.0,0.4}%
  \setlength{\shadeboxrule}{1.5pt}%
  \begin{definitions}[#1]\hspace*{1mm}%
}{\end{definitions}}

\begin{document}

  \begin{definitions}[Titel der Definition]
    Zugehöriger Text
  \end{definitions}

\end{document}

\documentclass{scrbook}

\usepackage{xcolor}
\usepackage[framemethod=tikz]{mdframed}
\usetikzlibrary{shadows}

\definecolor{mygray}{gray}{0.9}

\begin{document}
\mdtheorem[%
	backgroundcolor=mygray,%
	middlelinewidth=0pt,%
	roundcorner=4pt,%
	shadow=true,%
	skipbelow=15pt,%
	skipabove=15pt,%
]{definitions}{Definition}

\begin{definitions*}[Titel der Definition]
Zugehöriger Text
\end{definitions*}

\end{document}