%!TEX pdflatex

\documentclass[french,a4paper,12pt]{article}
\usepackage[most]{tcolorbox}
\usetikzlibrary{calc}
\usepackage{fontawesome5}

\tcbset{doublearrondi/.style={%
    enhanced,title={\bfseries{\small \faCommentDots[regular]} Définition : Écart-type},%
    coltitle=white,colback=blue!50!white,colbacktitle=blue!50!white,rounded corners,arc=8pt,colframe=blue!50!white,%
	underlay={%
	    \begin{tcbclipinterior}
	        \fill[draw=none,fill=white,rounded corners=8pt] ($(interior.south west)+(2pt,2pt)$) rectangle ($(interior.north east)+(-2pt,-2pt)$);
	    \end{tcbclipinterior}}%
	}%
}

\begin{document}

\begin{tcolorbox}[doublearrondi]
On appelle \textbf{écart-type} de la série le réel $\sigma=\sqrt{V}$.
\end{tcolorbox}

\end{document}