\documentclass[11pt]{scrartcl}
\usepackage{ntheorem}
\usepackage[ style =1 ,linewidth=0,linecolor=blue,backgroundcolor=yellow,margin=0pt,leftmargin=0cm,rightmargin=0cm,xcolor=cmyk] {mdframed}
\usepackage { lipsum}
\usepackage{showframe}
\newtheorem{mdlemma}{Lemma}[section]

\newenvironment{lemma}{%
    \begin{mdframed}[ntheorem=true,margin=10,backgroundcolor=red!30,linecolor=cyan,fontcolor=blue!70!black,linewidth=0,rightmargin=0cm,leftmargin=0cm,roundcorner=5pt,leftmargin=20pt,rightmargin=20pt]% 
     \begin{mdlemma}%
     }{%
     \end{mdlemma}%
    \end{mdframed}%
}


\begin{document}
\begin{mdframed} [skipabove=1cm, outerlinewidth=4pt , middlelinewidth=2pt , innerlinewidth=1pt ,%
outerlinecolor=blue , middlelinecolor=yellow , innerlinecolor=red ,%
backgroundcolor=orange,roundcorner=10pt]
\lipsum[1]
\end{mdframed}

    \begin{lemma}Text
           \begin{equation}
               x+y=2
           \end{equation}
           \par\noindent\rule{\linewidth}{2pt}
    \end{lemma}%
\clearpage%
\begin{mdframed}%
Some Text with an first empy line. The is only text to fill the line. It has no sence.

Some Text with an first empy line. The is only text to fill the line. It has no sence.
\end{mdframed}

\clearpage
   \section{foo}
% % % % % % % % % % %
   \begin{lemma}
%        \lipsum[1]\lipsum[1]\lipsum[1]
        \begin{itemize}
          \item HALLO
        \end{itemize}
    \end{lemma}

   \begin{lemma}
      HALLO WELT!
   \end{lemma}
    \begin{lemma}\mbox{ }\par\noindent
               \rule{\linewidth}{4pt}
    \end{lemma}
    \begin{mdframed}
       \rule{\linewidth}{4pt}
     \end{mdframed}
    \begin{lemma}
          \lipsum[5]\lipsum[5]
          \lipsum[5]\lipsum[5]
    \end{lemma}
    \lipsum[1]%
    \begin{lemma}Text
           \begin{equation}
               x+y=2
           \end{equation}
           \par\noindent\rule{\linewidth}{2pt}
    \end{lemma}%
%    
        \begin{lemma}\mbox{ }
                   \begin{equation}
                       x+y=2
                   \end{equation}
                   \par\noindent\rule{\linewidth}{2pt}
            \end{lemma}
    \begin{mdframed}%
           \lipsum[1]\par\noindent\rule{\linewidth}{2pt}
    \end{mdframed}

\end{document}
