This is what I'm using, @David.
\usepackage{thmtools}
\usepackage{mdframed}
\usepackage{blindtext}
\definecolor{mybg}{rgb}{0.9,0.9,0.9}
\newmdtheoremenv [backgroundcolor=mybg, %
innertopmargin =8pt , %
splittopskip = \topskip, %
skipbelow= 6pt, %
skipabove=6pt, %
topline=true,bottomline=false,leftline=false,rightline=false,]{substitutions}{Substitution}