Scusa hai ragione.
Allego un file minimale.
`\documentclass[a4paper, 11pt, openany, twoside]{book}
\usepackage[latin1]{inputenc}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage[T1]{fontenc}
\usepackage{libertine}
\usepackage[titles]{tocloft}
\renewcommand{\cftchapfont}{\sffamily \bfseries}
\renewcommand{\cftchappagefont}{\sffamily \bfseries}
\renewcommand{\cftsecfont}{\sffamily}
\renewcommand{\cftsecpagefont}{\sffamily}
%mdframed
\usepackage[framemethod=tikz]{mdframed}
\definecolor{mygrayone}{gray}{0.9}
\definecolor{mygraytwo}{gray}{0.5}
\newenvironment{myBg}
{\begin{mdframed}[innertopmargin=10pt, innerbottommargin=10pt, backgroundcolor=mygrayone,linecolor=mygraytwo,linewidth=0pt,outerlinewidth=0pt]}
{\end{mdframed}}
\begin{document}
\frontmatter
\tableofcontents
\mainmatter
\chapter{Il mio fedele cane}
\section{prima sezione}
\begin{myBg}
\lipsum[1]
\end{myBg}
\section{Seconda}
\chapter{Secondo capitolo della trattazione}
\backmatter
\end{document}`
[attachment=307]prova.tex[/attachment]