preamble_style.sty

% \usepackage[utf8]{inputenc}
\usepackage[a4paper, headheight=14pt, left=1.5cm, right=1.5cm, top=2cm, bottom=2cm]{geometry}
\usepackage{fancyhdr} % for customizing page-style (mainly headings).
\usepackage{titlesec} % for changing style of chapters, section... (with title format).
\usepackage{bookmark} % solve some warnings such as writing a section just below a part (i.e. without a chapter between them).
\usepackage[es-ES]{datetime2} % for writing the current date when compiling.
\usepackage{subfiles} % for creating subfiles .tex and compiling them separately to main.tex.
\usepackage{lmodern} % modern latin font. Also for avoiding (and correcting) some warnings. 
\usepackage{titlecaps}

%%%%%%%% Index %%%%%%%%%
\usepackage{makeidx}  % note the use of \makeindex in the preamble_style.sty
% \indexsetup{headers={\indexname}{\indexname}}
% \let\emph\relax % there's no \RedeclareTextFontCommand
% \DeclareTextFontCommand{\emph}{\bfseries\em}
% \newcommand\capmystring[1]{\capmystringaux#1\relax}
% \def\capmystringaux#1#2\relax{\uppercase{#1}#2} % only uppercase the text, ignore math
% % \renewcommand{\emph}[1]{{\em#1\index{\capmystring{#1}}}}
% % \usepackage{textcase} % in order to use \MakeUppercase
% % \newcommand{\ucfirst}[1]{\MakeUppercase{\f@rtocap{#1}}} % only capitalize the first letter
% % \def\f@rtocap#1\@nil{\uppercase{#1}\@nil}
% \usepackage{mfirstuc}
\renewcommand{\emph}[1]{{\em#1\index{#1}}}
%%%%%%%%%%%%%%%%%%%%%%%%

% %%%%%% For cross-refering among documents %%%%%%
% \usepackage{xr}
% \externaldocument{Mathematics/1st/Fundamentals_of_mathematics/Fundamentals_of_mathematics}
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%% Title format and page style %%%%%%
\titleformat{\chapter}[hang]{\normalfont\Huge\bfseries}{\thechapter.}{20pt}{}
\titleformat{\section}[block]
{{\color{\col}\titlerule[1.5pt]}\addvspace{4pt}\normalfont\fontsize{19}{21}\bfseries}
{\thesection.}      % label
{1em}   % separation between label and body
{}      % code before title body
[\vspace{2pt}{\color{\col}\titlerule[1.5pt]}] % code after title body 
\titleformat{\subsection}[hang]{\normalfont\fontsize{13}{15}\bfseries}{\thesubsection.\hspace{6pt}$\textcolor{\col}{\vf{|}}$\hspace{-6pt}}{1em}{}
\titleformat{\subsubsection}[hang]{\normalfont\fontsize{11}{13}\bfseries}{}{0pt}{}

% For well cross reference in the table of contents
\renewcommand*{\theHchapter}{\the\value{part}.\the\value{chapter}}
\renewcommand*{\theHsection}{\the\value{part}.\the\value{chapter}.\the\value{section}}
\renewcommand*{\theHsubsection}{\the\value{part}.\the\value{chapter}.\the\value{section}.\the\value{subsection}}

\ifSubfilesClassLoaded{% show only when the single subfile is compiled        
  \titleformat{\section}[block]
  {{\color{\col}\titlerule[1.5pt]}\addvspace{4pt}\normalfont\fontsize{19}{22}\bfseries}
  {}      % label
  {0pt}   % separation between label and body
  {}      % code before title body
  [\vspace{2pt}{\color{\col}\titlerule[1.5pt]}] % code after title body     

  \renewcommand{\thesection}{\arabic{section}}
  \renewcommand{\thesubsection}{\arabic{subsection}}

  % For numbering by default level-3 headings (i.e. subsubsections). It is not by default because we are in a report class document.
  \setcounter{secnumdepth}{3}
  % For creating references in the .pdf file.
  \setcounter{tocdepth}{3}


  \titlelabel{\thesubsubsection} % removing space between number and text (i.e. "1.  Probability" -----> "1.Probability").
  \renewcommand{\thesubsubsection}{} % removing the label of the subsubsection.

}{% show only when the main file is compiled
  \numberwithin{theorem}{section}
  \numberwithin{equation}{section} % resets equation numbering when starting a new section.

  %%% Page-style (heading) %%%
  \renewcommand{\chaptermark}[1]{\markboth{\thechapter.\ #1}{}}
  \pagestyle{fancy}
  \fancyhf{}
  \fancyhead[L]{\nouppercase{\leftmark}}
  \fancyhead[R]{\nouppercase{\rightmark}}
  \fancyfoot[C]{\thepage}
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%

  \makeindex
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%