\documentclass[11pt,fleqn]{book} % Default font size and left-justified equations
\usepackage[all]{xy}
\usepackage{amsmath,amsthm,amssymb,graphicx,tikz,fancyhdr,hyperref} 
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\usepackage[top=3cm,bottom=3cm,left=3.2cm,right=3.2cm,headsep=10pt,a4paper]{geometry} % Page margins

\usepackage{xcolor} % Required for specifying colors by name
\definecolor{ocre}{RGB}{243,102,25} % Define the orange color used for highlighting throughout the book
%\usepackage[framemethod=default]{mdframed}
\RequirePackage[framemethod=default]{mdframed}


\usepackage{graphicx} % Required for including pictures
\graphicspath{{Pictures/}} % Specifies the directory where pictures are stored

\usepackage{lipsum} % Inserts dummy text
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage{tikz} % Required for drawing custom shapes

%\usepackage[english]{babel} % English language/hyphenation

\usepackage{enumitem} % Customize lists
\setlist{nolistsep} % Reduce spacing between bullet points and numbered lists

\usepackage{booktabs} % Required for nicer horizontal rules in tables

%\usepackage{eso-pic} % Required for specifying an image background in the title page
\usepackage{xepersian}
%\usepackage[english,farsi]{babel}
\settextfont[Scale=1.2]{B Zar}
\setdigitfont[Scale=1.2]{B Zar}
\setlatintextfont{Arial}

%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%
\newtheoremstyle{examplestyle}
  {10mm}% measure of space to leave above the theorem. E.g.: 3pt
  {10mm}% measure of space to leave below the theorem. E.g.: 3pt
  {\slshape} % name of font to use in the body of the theorem
  {6pt}% measure of space to indent
{\bfseries\color{ocre}}
  %{\bfseries}% name of head font
  {}% punctuation between head and body
  {5mm}% space after theorem head
  {}
  %%%%%%%%%%%%%%%%
  %%%%%%%%%%%%%%%%%
  %%%%%%%%%%%%%%
\theoremstyle{examplestyle}\newtheorem{de}{تعریف}[section]
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skipbelow=7pt,
rightline=false,
leftline=true,
topline=false,
bottomline=false,
linecolor=ocre,
innerleftmargin=5pt,
innerrightmargin=5pt,
innertopmargin=0pt,
leftmargin=0cm,
rightmargin=0cm,
linewidth=4pt,
innerbottommargin=0pt]{dBox}
\newenvironment{examplestyle}{\begin{tBox}\begin{de}}{\end{de}\end{tBox}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%5

\begin{document}

سلام
\begin{de}
این یک تعریف است.
\end{de}

\end{document}