I agree that it is a useful feature to format the descriptive text with different font and color. I adopted the helpful answers of Gonzalo Medina and egreg and integrated some modification into the develpment code of tcolorbox
. Is is part of version 3.00 (2014/05/08)
.
%-- modification usable for tcolorbox
%-- version 2.70 (2014/02/06) to version 2.80 (2014/03/31)
%-- integrated into version 3.00 (2014/05/08) and above
\makeatletter
\def\tcb@theo@desc@form@std#1{\tcb@desc@col\kvtcb@desc@font\kvtcb@desc@delim@left#1\kvtcb@desc@delim@right}
\def\tcb@theo@title#1#2#3{%
\ifdefempty{#2}{\setbox\z@=\hbox{#1}}{\setbox\z@=\tcb@theo@form{#1}{#2}}%
\def\temp@a{#3}%
\ifx\temp@a\@empty\relax%
\unhbox\z@\kvtcb@terminatorsign%
\else%
\setbox\z@=\hbox{\unhbox\z@\kvtcb@separatorsign\ }%
\hangindent\wd\z@%
\hangafter=1%
\mbox{\unhbox\z@}{\tcb@theo@desc@form{#3}}\kvtcb@terminatorsign%
\fi%
}
\tcbset{
description color/.code={\def\temp@a{#1}\ifx\temp@a\@empty\relax%
\def\tcb@desc@col{}\else\def\tcb@desc@col{\color{#1}}\fi},
description color/.default=,
description font/.store in=\kvtcb@desc@font,
description font/.default=,
description formatter/.code={\let\tcb@theo@desc@form=#1},
description formatter/.default={\tcb@theo@desc@form@std},
description formatter,description color,description font
}
\makeatother
%-- end of modification
EDIT: The options are applied to your example like this:
\documentclass [10pt, twoside, openright, a5paper, showtrims]{memoir}
\usepackage{tcolorbox}
\tcbuselibrary{theorems,skins,breakable}
%-- insert modification from above if using tcolorbox
%-- version 2.70 (2014/02/06) to version 2.80 (2014/03/31)
%-- NOT needed for version 3.00 (2014/05/08) and above
\newtcbtheorem[auto counter,number within=chapter]{myDefinition}{Definition}{
unbreakable,
enhanced,
arc=0pt,
outer arc=0pt,
coltitle=white,
fonttitle=\small\bfseries,
description font=\normalfont,
left=7pt,
boxsep=2pt,
right=7pt,
}{def}
\begin{document}
\chapter{Intro}
\chapter{Basic}
\begin{myDefinition}{Contraction}{co-simulation}
some text
\end{myDefinition}
\end{document}

And here is the documentation of the new options:
