I suggest one of my favourite packages: tcolorbox
\documentclass{article}
\usepackage{tcolorbox}
% A new `\tcbox` command
\newtcbox{\highlight}[0]{boxsep=0pt,left=0pt,top=0pt,bottom=0pt,right=0pt,boxrule=0pt,arc=0pt,auto outer arc,colback=green,width=6cm}
\begin{document}
\highlight{black text on a boxed green background}
\begin{tcolorbox}[width=6cm,colback=green]
black text on a boxed green background
\end{tcolorbox}
\begin{tcolorbox}[width=6cm,colback=green,boxrule=5pt]
black text on a boxed green background
\end{tcolorbox}
\begin{tcolorbox}[width=6cm,colback=green,arc=0pt,auto outer arc]
black text on a boxed green background
\end{tcolorbox}
\end{document}

tcolorboxpackage. – egreg Apr 20 '15 at 18:11soulandulempackages were written to highlight (underline, background color, etc.) flowing text and might do something similar what you want. Don't think they can create exactly the same style like\fcolorbox, through. – Martin Scharrer Apr 20 '15 at 18:48