\documentclass[
a4paper,
twoside,
12pt,
french]{book}
%
% - Input encoding and font encoding -%
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
%
% - Page geometry -%
\usepackage[top=2.5cm,bottom=2.5cm,left=2.5cm,right=2.5cm]{geometry}
\usepackage{xcolor}
\usepackage{lipsum}
\makeatletter
\def\@makechapterhead#1{%
\vspace*{0\p@}% margin above chapter number
{\parindent \z@ \centering \normalfont\sffamily
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\Huge\bfseries\color{green} \@chapapp~\thechapter\\[-7pt]
% \par\nobreak
% \vskip 0\p@% space between chapter number and chapter title
\color{black}\rule{0.9\linewidth}{0.2pt}
\fi\\[7pt]
\fi
% \interlinepenalty\@M
\LARGE\bfseries\color{black} \centering #1
% \par\nobreak
\vskip 30 \p@% space below chapter title
}}
\makeatother
\begin{document}
\frontmatter
\mainmatter
\chapter{Ceci est un chapitre de test}
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