\usepackage[dvipsnames]{xcolor}
\usepackage{ifthen}
\usepackage[framemethod=tikz]{mdframed}

\newcommand{\linedenvTop}[2]
    {\begin{samepage}\qquad\textbf{#1}\ifthenelse{\equal{#2}{}}{}{ \textbf{(}\emph{#2}\textbf{)}}\trivlist\item[]\ignorespaces}
\newcommand{\linedenvBot}{\endtrivlist\end{samepage}}

\newenvironment{lemma}[1][]{\linedenvTop{Lemme}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=1.5pt,
    linecolor=BurntOrange,
    bottomline=false,topline=false,rightline=false]{lemma}

\newenvironment{definition}[1][]{\linedenvTop{Définition}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=1.5pt,
    linecolor=Plum,
    bottomline=false,topline=false,rightline=false]{definition}

\newenvironment{theorem}[1][]{\linedenvTop{Théorème}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=2.5pt,
    linecolor=Red,
    bottomline=false,topline=false,rightline=false]{theorem}

\definecolor{propositionRed}{HTML}{BE0000}
\newenvironment{prop}[1][]{\linedenvTop{Proposition}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=1.5pt,
    linecolor=propositionRed,
    bottomline=false,topline=false,rightline=false]{prop}

%\newenvironment{proof}{\textbf{Proof}\\}{}
\surroundwithmdframed[linewidth=1.0pt,
    linecolor=Blue,
    bottomline=false,topline=false,rightline=false]{proof}

\newenvironment{notation}[1][]{\linedenvTop{Notation}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=1.5pt,
    linecolor=Brown,
    bottomline=false,topline=false,rightline=false]{notation}

\newenvironment{example}[1][]{\linedenvTop{Exemple}{#1}}{\linedenvBot}
\surroundwithmdframed[linewidth=1.5pt,
    linecolor=LimeGreen,
    bottomline=false,topline=false,rightline=false]{example}