\documentclass[% Options of scrbook
  % draft,
  fontsize=12pt,
  %smallheadings,
  headings=big,
  english,
  paper=a4,
  twoside,
  open=right,
  DIV=14,
  BCOR=20mm,
  headinclude=false,
  footinclude=false,
  mpinclude=false,
  pagesize,
  titlepage,
  parskip=half,
  headsepline,
  chapterprefix=false,
  appendixprefix=Appendix,
  appendixwithprefixline=true,
  bibliography=totoc,
  toc=graduated,
  numbers=noenddot,
]{scrbook}

%
% Fold usepackages
%
\usepackage[
  ...
]{test}
\usepackage[
  backend=biber,
  style=numeric-comp,
  maxcitenames=99,
  doi=false,
  url=false,
  giveninits=true,
]{biblatex}

%
% Fold newcommands and similar
%
\renewcommand{\marginpar}{%
  \marginnote%
}
\newcommand*{\StoreCiteField}[3]{%
  \begingroup
    \global\let\StoreCiteField@Result\relax
    \citefield{#2}[StoreCiteField]{#3}%
  \endgroup
  \let#1\StoreCiteField@Result
}
\newenvironment{theo}[1][]{%
  \stepcounter{theo}%
  \ifstrempty{#1}{%
    \mdfsetup{
      frametitle={%
      \tikz[baseline=(current bounding box.east),outer sep=0pt]%
        \node[anchor=east,rectangle,fill=blue!20] {\strut Theorem~\thetheo};%
      }
    }%
  }%
  {% else ifstrempty:
  \mdfsetup{
    frametitle={
    \tikz[baseline=(current bounding box.east),outer sep=0pt]%
      \node[anchor=east,rectangle,fill=blue!20]{\strut Theorem~\thetheo:~#1};}%
  }
  }%
  \mdfsetup{
    innertopmargin=10pt,
    linecolor=blue!20,
    linewidth=2pt,topline=true,
    frametitleaboveskip=\dimexpr-\ht\strutbox\relax,
  }
  \begin{mdframed}[]\relax%
}{%
  \end{mdframed}%
}

\begin{document}

Hello World

\end{document}