Merci beaucoup de poser une question liée à `beamer` !
On pourrait faire les ajustements suivants (un peu compliqué, car ce thème normalement n'utilise pas de chiffres pour les sous-sections et les sous-sous-sections) :
\documentclass[french,t]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
\usepackage{babel}
\usetheme[hideothersubsections]{PaloAlto}
\title{Titre de la présentation}
\subtitle{Sous-titre de la présentation}
\author{Prénom \textsc{Nom}}
\institute{Institut}
\date{Date}
\makeatletter
\setbeamertemplate{section in toc}{\leavevmode\leftskip=2.75ex%
\llap{%
\normalsize%
\begin{pgfpicture}{-1ex}{-0.7ex}{1ex}{1ex}
\pgftext{\beamer@usesphere{section number projected}{tocsphere}}
\pgftext{%
\usebeamerfont*{section number projected}%
\usebeamercolor{section number projected}%
\color{fg!90!bg}%
\@Alph\inserttocsectionnumber}
\end{pgfpicture}%
\kern1.25ex}%
\inserttocsection\par
}
\pgfdeclareradialshading[bg,parent.bg]{tocsmallsphere}{\pgfpoint{-0.4ex}{0.48ex}}%
{%
color(0cm)=(bg!35!white);
color(0.352ex)=(bg!75!white);
color(0.704ex)=(bg!70!black!90!parent.bg);
color(0.96ex)=(bg!50!black!90!parent.bg);
color(1.04ex)=(parent.bg)}
\setbeamerfont{subsection number projected}{size=\tiny}
\setbeamertemplate{subsection in toc}{%
\leavevmode\leftskip=6ex%
\llap{%
\normalsize%
\begin{pgfpicture}{-0.7ex}{-0.7ex}{0.7ex}{0.7ex}
\pgftext{\beamer@usesphere{subsection number projected}{tocsmallsphere}}
\pgftext{%
\usebeamerfont*{subsection number projected}%
\usebeamercolor{subsection number projected}%
\color{fg!90!bg}%
\inserttocsubsectionnumber}
\end{pgfpicture}%
\kern1.25ex}%
\inserttocsubsection\par
}
\pgfdeclareradialshading[bg,parent.bg]{toctinysphere}{\pgfpoint{-0.32ex}{0.384ex}}%
{%
color(0cm)=(bg!35!white);
color(0.2816ex)=(bg!75!white);
color(0.5632ex)=(bg!70!black!90!parent.bg);
color(0.768ex)=(bg!50!black!90!parent.bg);
color(0.832ex)=(parent.bg)}
\setbeamerfont{subsubsection number projected}{size=\tiny}
\setbeamertemplate{subsubsection in toc}{%
\leavevmode\leftskip=11ex%
\llap{%
\normalsize%
\begin{pgfpicture}{-0.5ex}{-0.45ex}{0.5ex}{0.5ex}
\pgftext{\beamer@usesphere{subsubsection number projected}{toctinysphere}}
\pgftext{%
\usebeamerfont*{subsubsection number projected}%
\usebeamercolor{subsubsection number projected}%
\color{fg!90!bg}%
\@alph\inserttocsubsubsectionnumber
}
\end{pgfpicture}%
\kern1.25ex}%
\inserttocsubsubsection\par
}
\makeatother
% Table des matières qui apparaît au début de chaque division :
\AtBeginSection{%
\begin{frame}
\frametitle{Plan}
\tableofcontents[sectionstyle=show/shaded,subsectionstyle=hide,subsubsectionstyle=hide]
\end{frame}
}
\begin{document}
\begin{frame}
\frametitle{Plan}
\tableofcontents
\end{frame}
\section{Section 1}
\subsection{Subsection 1}
\subsubsection{subsubsection 1}
\begin{frame}
\frametitle{Titre}
\framesubtitle{Sous-titre}
\end{frame}
\begin{frame}
\frametitle{Titre blabla}
Blablabla
\end{frame}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blibli}
Bliblibli
\end{frame}
\section{Section 2}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blabla}
Blablabla
\end{frame}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blibli}
Bliblibli
\end{frame}
\section{Conclusion}
\begin{frame}
\frametitle{Conclusion}
C'est la fin
\end{frame}
\end{document}
![alt text][1]
Sans les boules :
\documentclass[french,t]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
\usepackage{babel}
\usetheme[hideothersubsections]{PaloAlto}
\title{Titre de la présentation}
\subtitle{Sous-titre de la présentation}
\author{Prénom \textsc{Nom}}
\institute{Institut}
\date{Date}
\makeatletter
\setbeamertemplate{section in toc}{\leavevmode\@Alph\inserttocsectionnumber. \inserttocsection\par}
\setbeamertemplate{subsection in toc}{\leavevmode\normalsize\usebeamerfont{subsection in
toc}\leftskip=2.4em\usebeamerfont{subsection in toc}\rlap{\hskip-1.2em\inserttocsubsectionnumber. }\inserttocsubsection\par}
\setbeamertemplate{subsubsection in toc}{\leavevmode\normalsize\usebeamerfont{subsubsection in
toc}\leftskip=4.2em\usebeamerfont{subsubsection in toc}%
\rlap{\hskip-1.2em\@alph\inserttocsubsubsectionnumber. }\inserttocsubsubsection\par}
\makeatother
% Table des matières qui apparaît au début de chaque division :
\AtBeginSection{%
\begin{frame}
\frametitle{Plan}
\tableofcontents[sectionstyle=show/shaded,subsectionstyle=hide,subsubsectionstyle=hide]
\end{frame}
}
\begin{document}
\begin{frame}
\frametitle{Plan}
\tableofcontents
\end{frame}
\section{Section 1}
\subsection{Subsection 1}
\subsubsection{subsubsection 1}
\begin{frame}
\frametitle{Titre}
\framesubtitle{Sous-titre}
\end{frame}
\begin{frame}
\frametitle{Titre blabla}
Blablabla
\end{frame}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blibli}
Bliblibli
\end{frame}
\section{Section 2}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blabla}
Blablabla
\end{frame}
\subsection{Subsection 2}
\begin{frame}
\frametitle{Titre blibli}
Bliblibli
\end{frame}
\section{Conclusion}
\begin{frame}
\frametitle{Conclusion}
C'est la fin
\end{frame}
\end{document}
![alt text][2]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-04-15_at_15.58.09.png
[2]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-04-15_at_16.08.17.png