\usepackage{tikz,environ}
\tikzstyle{mybox} = [draw=black, very thick,
    rectangle, inner sep=10pt, inner ysep=20pt]
\tikzstyle{fancytitle} =[draw=black, fill=white, text=black]


\NewEnviron{defin}{\par
\begin{center}
\begin{tikzpicture}
\node [mybox] (box){%
    \begin{minipage}{0.8\textwidth}
    	\BODY
	\end{minipage} }; 
\node[fancytitle, right=10pt] at (box.north west) {\textbf{Définition}};
\end{tikzpicture}\end{center}\par
}

\NewEnviron{thm}{\par
\begin{center}
\begin{tikzpicture}
\node [mybox] (box){%
    \begin{minipage}{0.8\textwidth}
    	\BODY
	\end{minipage} }; 
\node[fancytitle, right=10pt] at (box.north west) {\textbf{Théorème}};
\end{tikzpicture}\end{center}\par
}

\usepackage{thmbox}
\usepackage{xparse}
\NewDocumentEnvironment{prop}{o}{%
\begin{center}\begin{minipage}{0.8\textwidth}
  \thmbox[M]{\textbf{Proposition \IfNoValueTF{#1}{}{\em (#1)}}}%
  %\hspace*{-1.5em}%
  }
  {%
  \endthmbox\vspace*{.75ex}\end{minipage}\end{center}%
  }



\NewDocumentEnvironment{corol}{o}{%
	\begin{center}\begin{minipage}{0.8\textwidth}
			\thmbox[M]{\textbf{Corollaire \IfNoValueTF{#1}{}{\em (#1)}}}%
			%\hspace*{-1.5em}%
		}
		{%
			\endthmbox\vspace*{.75ex}\end{minipage}\end{center}%
}

\NewDocumentEnvironment{lemEM}{o}{%
	\begin{center}\begin{minipage}{0.8\textwidth}
			\thmbox[M]{\textbf{Lemme ergodique maximal \IfNoValueTF{#1}{}{\em (#1)}}}%
			%\hspace*{-1.5em}%
		}
		{%
			\endthmbox\vspace*{.75ex}\end{minipage}\end{center}%
}

\newenvironment{rem}{\par\textbf{Remarque: }\nopagebreak}{\par}
\newenvironment{exemple}{\par\textbf{Exemple: }\nopagebreak}{\par}
\newenvironment{preuve}{\par\textit{Démonstration: }\nopagebreak}{\par}