答案1
像这样吗?
\documentclass{article}
\usepackage{amsthm}
\usepackage[skins]{tcolorbox}
\newtcolorbox{myframe}{
enhanced,
sharp corners=all,
colback=white,
colframe=orange,
toprule=0pt,
bottomrule=0pt,
leftrule=1.5pt,
rightrule=0pt,
overlay={\draw[orange,line width=1.5pt] (frame.south west) -- ++(1cm,0pt);}
}
\theoremstyle{definition}
\newtheorem{definition}{Definition}
\begin{document}
\begin{myframe}
\begin{definition}
Definition
\end{definition}
\begin{enumerate}
\item a
\item b
\item c
\end{enumerate}
\end{myframe}
\end{document}