\documentclass[bibliography=totocnumbered]{scrartcl}
\usepackage{tabulary}
\usepackage{graphicx}
\usepackage[ngerman]{babel}
\usepackage{booktabs}
\usepackage{amssymb, amsthm, mathtools}
\usepackage[printonlyused, withpage]{acronym}
\usepackage[parfill]{parskip}
\usepackage{fancyhdr}
\usepackage{wrapfig}
\usepackage[numbers]{natbib}
\bibliographystyle{plainnat}
\usepackage[table]{xcolor} %for more colors (used in url and links)
\definecolor{links}{HTML}{1a1a1a}
\usepackage{hyperref}
\hypersetup{
    colorlinks=true, %set true if you want colored links
    linktoc=all,     %set to all if you want both sections and subsections linked
    linkcolor=black,  %choose some color if you want links to stand out
    urlcolor=links,
    citecolor=blue
}

%Time in right format
\usepackage[yyyymmdd]{datetime}
\renewcommand{\dateseparator}{-}

\usepackage{url}                % Lets you typeset urls. Usage: \url{http://...}
\usepackage{breakurl}           % Enables linebreaks for urls
\urlstyle{same}
%No end dot in nummbering
%\renewcommand{\autodot}{}% Remove all end-of-counter dots

%For custom font
%\usepackage{fontspec}
%\setmainfont{Cambria Math} %requires package fontspec

\usepackage[a4paper, left=2.5cm, right=3cm, top=2.5cm, bottom=2cm]{geometry}

%Framed boxes
\usepackage[framemethod=TikZ]{mdframed}

\newcounter{lem}[section]\setcounter{lem}{0}
\renewcommand{\thelem}{\arabic{section}.\arabic{lem}}

\newenvironment{lem}[2][]{%
\refstepcounter{lem}%
\ifstrempty{#1}%
{\mdfsetup{%
frametitle={%
\tikz[baseline=(current bounding box.east),outer sep=0pt]
\node[anchor=east,rectangle,fill=blue!20]
{\strut Lemma~\thelem};}}
}%
{\mdfsetup{%
frametitle={%
\tikz[baseline=(current bounding box.east),outer sep=0pt]
\node[anchor=east,rectangle,fill=blue!20]
{\strut #1};}}%
}%
\mdfsetup{innertopmargin=10pt,linecolor=blue!20,%
linewidth=2pt,topline=true,%
frametitleaboveskiip=\dimexpr-\ht\strutbox\relax}
\begin{mdframed}[]\relax%
\label{#2}}{\end{mdframed}}

\newcommand{\HRule}{\rule{\linewidth}{0.5mm}}

\begin{document}
\begin{center}
\begin{tabulary}{\textwidth}{|C|}
\toprule
WILHELM-DIESS-GYMNASIUM
POCKING\\
\midrule
Qualifikationsphase 2020/2022\\
\cellcolor[HTML]{111111}SEMINARARBEIT\\
\end{tabulary}\\

\end{center}
\end{document}