Voici une autre solution (plus simple), qui répond à la question posée en commentaires.
\documentclass[
mainlanguage=english, french,
localtocs,
localbibs*,
secnumdepth=subsubsection,
titlesec={pagestyles}]{yathesis}
\usepackage{lipsum}
\usepackage{geometry}
\makeatletter
\def\@makechapterhead#1{%
\vspace*{0\p@}% (no extra top margin)
{\parindent \z@ \centering \normalfont\sffamily
% --- Chapter number line with colored text and rule ---
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\Huge\bfseries\color{gray}\@chapapp~\thechapter\par\nobreak
\vskip -7\p@ % tighten spacing above the rule
{\color{black}\rule{0.9\linewidth}{0.2pt}}\par % horizontal rule
\vskip 7\p@ % space between rule and title
\fi
\fi
% --- Chapter title line ---
\LARGE\bfseries\color{blue} #1\par}%
\vskip 30\p@ % space below chapter title
% --- Insert leadchapter text, if provided ---
\@ifundefined{YAD@leadchapter}{}{%
\noindent\YAD@leadchapter\par
\let\YAD@leadchapter\relax
}%
% --- Insert local chapter TOC, if enabled ---
\ifbool{YAD@localtocs}{%
\YAD@localtoc
\renewcommand{\YAD@localtoc}{\YAD@default@localtoc}%
\ifbool{YAD@nextwithlocaltoc}{%
\boolfalse{YAD@localtocs}\boolfalse{YAD@nextwithlocaltoc}}{}%
% (no extra gap here; adjust with \vskip if desired)
}{}%
\@afterheading
\clearpage
}
\def\@makeschapterhead#1{%
\vspace*{0\p@}% (no extra top margin)
{\parindent \z@ \centering \normalfont\sffamily
\LARGE\bfseries\color{red} #1\par}%
\vskip 30\p@ % space below chapter title
% --- Insert leadchapter text, if provided ---
\@ifundefined{YAD@leadchapter}{}{%
\noindent\YAD@leadchapter\par
\let\YAD@leadchapter\relax
}%
% --- Insert local chapter TOC, if enabled ---
\ifbool{YAD@localtocs}{%
\YAD@localtoc
\renewcommand{\YAD@localtoc}{\YAD@default@localtoc}%
\ifbool{YAD@nextwithlocaltoc}{%
\boolfalse{YAD@localtocs}\boolfalse{YAD@nextwithlocaltoc}}{}%
% (no extra gap here; adjust with \vskip if desired)
}{}%
\@afterheading
}\let\old@makechapterhead\@makechapterhead
\AfterPreamble{%
\patchcmd{\chapter}{{\YAD@starred@chapter}{\YAD@unstarred@chapter}}{{\let\@makechapterhead\@makeschapterhead\YAD@starred@chapter}{\let\@makechapterhead\old@makechapterhead\YAD@unstarred@chapter}}{}{\err}
}
\makeatother
\geometry{
paper=a4paper
}
%
\begin{document}
\tableofcontents[depth=section]
\mainmatter
\chapter{Le premier}
\lipsum{4}
\chapter{Le deuxième}
\lipsum{8}
\chapter*{Un test non num}
\lipsum{6}
\end{document}
Voici une solution.
\documentclass[
mainlanguage=english, french,
localtocs,
localbibs*,
secnumdepth=subsubsection,
titlesec={pagestyles}]{yathesis}
\usepackage{lipsum}
\usepackage{geometry}
\makeatletter
\patchcmd{\YAD@globalstyle}{\chapter*}{\chapter*[]}{}{\err}
\AfterPreamble{%
\RenewDocumentCommand{\YAD@tmp@chapter}{sO{}O{}m}{%
\ifthenelse{\isempty{#2}}{%
\ifthenelse{\isempty{#3}}{%
\IfBooleanTF{#1}{\YAD@ORI@chapter*{#4}}{\YAD@ORI@chapter{#4}}%
}{%
\IfBooleanTF{#1}{\YAD@ORI@chapter*{#4}}{\YAD@ORI@chapter{#4}}%
\chaptermark{#3}%
}%
}{%
\ifthenelse{\isempty{#3}}{%
\IfBooleanTF{#1}{\YAD@ORI@chapter*[#2]{#4}}{\YAD@ORI@chapter[#2]{#4}}%
}{%
\IfBooleanTF{#1}{\YAD@ORI@chapter*[#2]{#4}}{\YAD@ORI@chapter[#2]{#4}}%
\chaptermark{#3}%
}%
}%
}
\RenewDocumentCommand{\YAD@starred@chapter}{O{}O{}m}{%
\booltrue{YAD@nonnumbered@chapter}%
\booltrue{YAD@nonnumbered@section}%
\booltrue{YAD@nonnumbered@subsection}%
\booltrue{YAD@nonnumbered@subsubsection}%
\booltrue{YAD@nonnumbered@paragraph}%
\booltrue{YAD@nonnumbered@subparagraph}%
\YAD@setsecnumdepth{none}%
\YAD@tmp@chapter*[#1][#2]{#3}%
}}
\def\@makechapterhead#1{%
\vspace*{0\p@}% (no extra top margin)
{\parindent \z@ \centering \normalfont\sffamily
% --- Chapter number line with colored text and rule ---
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\Huge\bfseries\color{gray}\@chapapp~\thechapter\par\nobreak
\vskip -7\p@ % tighten spacing above the rule
{\color{black}\rule{0.9\linewidth}{0.2pt}}\par % horizontal rule
\vskip 7\p@ % space between rule and title
\fi
\fi
% --- Chapter title line ---
\LARGE\bfseries\color{blue} #1\par}%
\vskip 30\p@ % space below chapter title
% --- Insert leadchapter text, if provided ---
\@ifundefined{YAD@leadchapter}{}{%
\noindent\YAD@leadchapter\par
\let\YAD@leadchapter\relax
}%
% --- Insert local chapter TOC, if enabled ---
\ifbool{YAD@localtocs}{%
\YAD@localtoc
\renewcommand{\YAD@localtoc}{\YAD@default@localtoc}%
\ifbool{YAD@nextwithlocaltoc}{%
\boolfalse{YAD@localtocs}\boolfalse{YAD@nextwithlocaltoc}}{}%
% (no extra gap here; adjust with \vskip if desired)
}{}%
\@afterheading
\clearpage
}
\def\@makeschapterhead#1{%
\vspace*{0\p@}% (no extra top margin)
{\parindent \z@ \centering \normalfont\sffamily
\LARGE\bfseries\color{red} #1\par}%
\vskip 30\p@ % space below chapter title
% --- Insert leadchapter text, if provided ---
\@ifundefined{YAD@leadchapter}{}{%
\noindent\YAD@leadchapter\par
\let\YAD@leadchapter\relax
}%
% --- Insert local chapter TOC, if enabled ---
\ifbool{YAD@localtocs}{%
\YAD@localtoc
\renewcommand{\YAD@localtoc}{\YAD@default@localtoc}%
\ifbool{YAD@nextwithlocaltoc}{%
\boolfalse{YAD@localtocs}\boolfalse{YAD@nextwithlocaltoc}}{}%
% (no extra gap here; adjust with \vskip if desired)
}{}%
\@afterheading
}
\makeatother
\geometry{
paper=a4paper
}
%
\begin{document}
\tableofcontents[depth=section]
\mainmatter
\chapter{Le premier}
\lipsum{4}
\chapter{Le deuxième}
\lipsum{8}
\chapter*{Un test non num}
\lipsum{6}
\end{document}