\documentclass{beamer}
\usetheme[progressbar=frametitle]{metropolis}
\setbeamertemplate{frame numbering}[fraction]
\useoutertheme{metropolis}
\useinnertheme{metropolis}
%\usefonttheme{Berlin}
\usecolortheme{orchid}
\setbeamercolor{background canvas}{bg=white}
\renewcommand{\thesection}{\Roman{section}.}
\renewcommand{\thesubsection}{\arabic{subsection}}
\renewcommand{\thesubsubsection}{\alpha{subsubsection}}
\usepackage{romannum}
\makeatletter
\setbeamertemplate{section in toc}{%
\leavevmode%
% prevents the period to be printed with the first/last section option
\ifnum\beamer@tempcount>\beamer@toclastsection
\else
\ifnum\beamer@tempcount>0
\Romannum{\inserttocsectionnumber}
\fi\fi%
\inserttocsection\par%
}
\makeatother
\setbeamertemplate{subsection in toc}[subsections numbered]
\defbeamertemplate{subsubsection in toc}{subsubsections numbered}
{\leavevmode\leftskip=3em%
\rlap{\hskip-3em\inserttocsectionnumber.\inserttocsubsectionnumber.\inserttocsubsubsectionnumber}%
\inserttocsubsubsection\par}
toc}{\leavevmode\leftskip=2em\rlap{\hskip-2em\Romannum{\inserttocsectionnumber}.\inserttocsubsectionnumber}\inserttocsubsection\par}
\setbeamertemplate{subsubsection in toc}[subsubsections numbered]
toc}
{\leavevmode\leftskip=3em%
\rlap{\hskip-3em\Romannum{\inserttocsectionnumber}.\inserttocsubsectionnumber.\@alph{\inserttocsubsubsectionnumber}}%
\inserttocsubsubsection\par}
\makeatother
\makeatletter
\setbeamertemplate{section page}{%
\centering
\begin{minipage}{22em}
\raggedright
\usebeamercolor[fg]{section title}
\usebeamerfont{section title}
\thesection~\insertsectionhead\\[-1ex]
\usebeamertemplate*{progress bar in section page}
\par
\ifx\insertsubsectionhead\@empty\else%
\usebeamercolor[fg]{subsection title}%
\usebeamerfont{subsection title}%
\insertsubsectionhead
\fi
\end{minipage}
\par
\vspace{\baselineskip}
}
\makeatother
\begin{document}
\begin{frame}{Titre}
\tableofcontents
\end{frame}
\section{Rappels}
\subsection{Fondements}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsubsection{Hypoth\`eses}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsubsection{Causes}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{Implications}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{Critiques}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\section{Th\'eorie}
\subsection{Plan}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{\'Equilibre}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/document_2PMCtZ3.pnghttps://texnique.fr/osqa/upfiles/document_Wj16oog.png
