为小节编号增加一个颜色盒子

代码实现如下

\documentclass{book}
\usepackage[explicit]{titlesec}
\usepackage{tikz}
\usepackage{lipsum}
\usepackage{fancyhdr}

\colorlet{sectcolor}{gray!65}

\newcommand\graysquare{\textcolor{sectcolor}{\rule{1ex}{1ex}}}
\newcommand{\helv}{\fontfamily{phv}\fontseries{}\fontsize{10}{12}\selectfont}

\pagestyle{fancy}

\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\renewcommand{\sectionmark}[1]{\markright{\thesection.\ #1}}
\fancyhf{}
\fancyhead[RO]{\helv\rightmark\hspace{0.5em}\graysquare\hspace{0.5em}\thepage}
\fancyhead[LE]{\helv\thepage\hspace{0.5em}\graysquare\hspace{0.5em}\leftmark}
\renewcommand{\headrulewidth}{0pt}

\titleformat{\section}
{\normalfont\Large\bfseries}
{\llap{\colorbox{sectcolor}{\makebox[3em][r] {\thesection}}\hspace{1em}}}
{0em}{#1}

\begin{document}

\setcounter{chapter}{1}% just for the example
\section{Introduction}
A cross-reference to the last section:~\ref{sec:conclusion}.
\lipsum[4]
\section{Discussion}
\lipsum[4]
\section{Conclusion}
\label{sec:conclusion}
\lipsum[4]

\end{document}

选自：http://tex.stackexchange.com/questions/100677/use-of-shaded-box-for-section-numbering-only