答案1
\documentclass{article}
\usepackage[most]{tcolorbox}
\begin{document}
\begin{tcolorbox}
My outer box
\begin{tcolorbox}
My inner box
\end{tcolorbox}
\end{tcolorbox}
\begin{tcolorbox}[left=1mm, right=1mm]
My outer box
\begin{tcolorbox}
My inner box
\end{tcolorbox}
\end{tcolorbox}
\begin{tcolorbox}
My outer box
\begin{tcolorbox}[enlarge left by=-4mm, enlarge right by=-4mm, width=\linewidth+8mm]
My inner box
\end{tcolorbox}
\end{tcolorbox}
\end{document}