\documentclass[12pt,a4paper,openany]{book}
\usepackage{tcolorbox}
\tcbuselibrary{theorems}
\newtcbtheorem
[]% init options
{definition}% name
{abc}
% title
{%
colback=green!5,
colframe=green!35!black,
fonttitle=\bfseries,
}% options
% {def}% prefix
{}
\begin{document}
\begin{definition}{}{}
zzz
\end{definition}
\end{document}