New environment. Highlight content with symbols and indentation

Question

I'm writing my thesis and I want to highlight some part of my text which resides in a separate paragraph. I like the well-known example of

\newenvironment{king}
{\rule{1ex}{1ex}\hspace{\stretch{1}}}
{\hspace{\stretch{1}}\rule{1ex}{1ex}}

and then

\begin{king}
My humble subjects \ldots
\end{king}

But if the sentence is very long (more than one line) the result is not good looking at all, e.g.

\begin{king}
Energy is the property of matter and radiation that is manifest as a capacity to perform work.
\end{king}

I'm wondering how I can keep the margins (left and right), repeat the square every line and.. make it bold (or with a gray background).

Thanks for any help Alex PS: This is my first post so I hope it's good

Answer 1

You can adapt an answer of mine:

\documentclass{article}
\usepackage{environ}
\usepackage{lipsum}

\makeatletter
\newif\ifboxended
\def\centervbox#1{\begingroup\global\setbox\@ne=\box\voidb@x
  \global\boxendedfalse
  \setbox\z@=\copy#1\relax\docentervbox}
\def\docentervbox{%
  \setbox\z@=\vbox{\unvbox\z@ 
    \ifcase\lastnodetype
% char node (can't remove)
    \or
% hlist node
    \setbox\tw@=\lastbox
    \def\next{%
      \global\setbox\@ne=\vbox{
        \hbox to\hsize{\rule{1ex}{1ex}\hfil\box\tw@\hfil\rule{1ex}{1ex}}\unvbox\@ne}}%
    \or
% vlist node
    \setbox\tw@=\lastbox
    \def\next{%
      \global\setbox\@ne=\vbox{
        \hbox to\hsize{\rule{1ex}{1ex}\hfil\box\tw@\hfil\rule{1ex}{1ex}}\unvbox\@ne}}%
    \or
% rule node (can't remove)
    \or
% ins node (can't remove)
    \or
% mark node (can't remove)
    \or
% adjust node (can't remove)
    \or
% ligature node (can't happen)
    \or
% disc node (can't happen)
    \or
% whatsit node (can't remove)
    \or
% math node (can't remove)
    \or
% glue node
    \skip@=\lastskip\unskip
    \def\next{\global\setbox\@ne=\vbox{\vskip\skip@\unvbox\@ne}}%
    \or
% kern node
    \dimen@=\lastkern\unkern
    \def\next{\global\setbox\@ne=\vbox{\kern\dimen@\unvbox\@ne}}%
    \or
% penalty node
    \count@=\lastpenalty\unpenalty
    \def\next{\global\setbox\@ne=\vbox{\penalty\count@\unvbox\@ne}}%
    \or
% unset node (can't happen)
    \or
% math mode node (can't remove)
    \else
% empty list
    \def\next{\global\boxendedtrue}
    \fi
    \next}
  \ifboxended
    \def\next{\unvbox\@ne\endgroup}
  \else
    \let\next\docentervbox
  \fi
  \next}
\makeatother
\NewEnviron{king}{%
  \trivlist\item\relax
  \setbox4=\vbox{\advance\hsize-4ex\centering\BODY\par}\centervbox{4}%
  \endtrivlist
}



\begin{document}

\lipsum[2]

\begin{king}
Energy is the property of matter and radiation that is manifest as a capacity to perform work.
\end{king}

\lipsum[2]

\begin{king}
My humble subjects
\end{king}

\lipsum[2]

\end{document}