I want to have a colored box around the text/sentences and its width will be fixed to be 80% of the page width. I tried \fcolorbox but it can't break lines. It is possible to do it with TikZ or is there a handy package available? Thx!

I want to have a colored box around the text/sentences and its width will be fixed to be 80% of the page width. I tried \fcolorbox but it can't break lines. It is possible to do it with TikZ or is there a handy package available? Thx!

with tcolorbox:
\documentclass{article}
\usepackage{tcolorbox}
\definecolor{myviolet}{rgb}{0.73,0.56,0.64}
\newtcolorbox{mybox}{
arc=0pt,
boxrule=0pt,
colback=myviolet,
width=.8\textwidth, % this option controls the width of the box
colupper=white,
fontupper=\bfseries
}
\begin{document}
\begin{mybox}
Example
\end{mybox}
\end{document}

It seems that an answer with mdframed is missing. Here it is.
\documentclass{article}
\usepackage[framemethod=TikZ]{mdframed}
\definecolor{myviolet}{rgb}{0.73,0.56,0.64}
\usepackage{lipsum}
\newmdenv[%
rightmargin=.2\textwidth,
backgroundcolor=myviolet,
linewidth=0pt,
fontcolor=white%
]{mybox}
\begin{document}
\lipsum[1]
\begin{mybox}
Example
\end{mybox}
\end{document}

Solution Idea
In order to break lines, we can use minipage environment inside a \colorbox.
The Solution
\documentclass{article}
\usepackage[usenames,dvipsnames]{color}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\medskip
\colorbox{Lavender}{
\begin{minipage}[c]{0.8\textwidth}
\large\color{White}\lipsum[2]
\end{minipage}}
\medskip
\lipsum[3]
\end{document}
Output

Limitation
Can not handle page breaks.
This seems to give what you want, at least as long as you do not need pagebreaks (for such needs look at the framed or mdframed packages):
\documentclass[12pt, a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[showframe,noheadfoot,nomarginpar,textwidth=15cm,textheight=23cm]{geometry}
\usepackage[x11names]{xcolor}
\begin{document}
\noindent\colorbox{Thistle3}{\parbox{\dimexpr 0.8\textwidth -2\fboxsep\relax} {\sffamily\bfseries\color{white}Example. Example. Example. Example. Example. Example. Example. Example.}}
\end{document}

\colorboxcommand, apply more broadly. – Mico Dec 27 '13 at 16:04