I have a box for tips, but I would like to have a pattern surrounding it like in the example picture.

Thank you for your help!
\documentclass[ngerman]{scrbook}
\usepackage{fontspec}
\usepackage{babel}
\usepackage{tabularx,colortbl,multirow,dcolumn}
\renewcommand\tabularxcolumn[1]{b{#1}}
\usepackage{xcolor}
\usepackage{calc}
\usepackage{titletoc,titlesec,enumitem}
\newenvironment{Test}{
\begin{center}
\begin{sffamily}
\setlength\arrayrulewidth{0.75pt}\arrayrulecolor{white}
\renewcommand{\arraystretch}{1.3}
\begin{tabular}[h!]{p{\textwidth-2\tabcolsep}}
\rowcolor[gray]{0.6} \textbf{{\color{white}Testbox}} \\
\hline
\rowcolor[gray]{0.90} }
{\\
\end{tabular}
\end{sffamily}
\end{center}
}
\begin{document}
\begin{Test}
Content
\end{Test}
\end{document}

