preamble_general.sty

\usepackage{amsthm, amsmath, mathtools, amssymb, physics} % for math and physics commands and symbols.
\usepackage[intlimits]{esint} % for more options on integrals.
\usepackage{standalone} % for compiling individually .tex images.
\usepackage[hypcap=false]{caption} % basically for using the command \captionof{}, which is useful in some environments, others than \begin{figure}.
\usepackage{subcaption} % for subfigures, when many captions come into play.
\usepackage{multicol} % organize the whole document in multiple columns.
\usepackage[USenglish]{babel} % for improving use of English and mainly the use of hyphenation.
\usepackage[version=4]{mhchem} % used for chemistry and nuclear physics.
\usepackage{multirow} % multirow in a table, for example.
\usepackage{array, makecell} % for additional options in tables and tabular.
\usepackage{hyperref} % link automatically references.
% \usepackage{hycolor} % implements options of colors for hyperref (it ables the user to use colors from xcolor package)
\usepackage[capitalise,nameinlink]{cleveref} % for clever references. That is, for example, for automatically write "Definition 2.13" instead of only "2.13".
\usepackage{graphicx} % for options like \definecolor{}{}{}, \textcolor{}{},...
\usepackage[usenames,x11names]{xcolor} % to customize RGB colors.
\usepackage{colortbl} % for colors in tables.
\usepackage{tcolorbox} % for coloring boxes.
\setlength{\parindent}{0pt} % Indentation disabled.

\usepackage{float} % to allow using argument [H] in figure environtment 

\setlength\multlinegap{0pt} % disable the margins on \begin{multline} command.
\allowdisplaybreaks % allows to break an align environment in two pages.

%% colors - subjects %%
\newcommand{\col}{red}%            % DEFAULT
\newcommand{\ana}{DeepSkyBlue2}%   % analysis
\newcommand{\alg}{Green3}%         % algebra
\newcommand{\geo}{Purple2}%        % geometry and topology
\newcommand{\apl}{Red2}%           % applied mathematics
\newcommand{\pro}{Orange2}%        % probability and statistics
\newcommand{\phy}{SlateBlue2}%     % physics

\ExplSyntaxOn
\NewExpandableDocumentCommand{\refcol}{m}{
  \str_case_e:nnF {\FirstWord{#1}}{
    {FM}{\alg}    % fundamentals of mathematics 
      {LA}{\alg}    % linear algebra
      {RVF}{\ana}   % real valued functions
      {AS}{\alg}    % algebraic structures
      {DM}{\alg}    % discrete mathematics
      {FSV}{\ana}   % functions of several variables
      {LG}{\geo}    % linear geometry
      {MA}{\ana}    % mathematical analysis
      {NM}{\apl}    % numerical methods
      {CA}{\ana}    % complex analysis
      {DE}{\apl}    % differential equations
      {DG}{\geo}    % differential geometry
      {GT}{\alg}    % Galois theory
      {P}{\pro}     % probability
      {S}{\pro}     % statistics
      {TOP}{\geo}   % topology
      {AT}{\geo}    % algebraic topology
      {A}{\alg}     % arithmetic
      {CAL}{\alg}   % commutative algebra
      {DS}{\apl}    % dynamical systems
      {HA}{\ana}    % harmonic analysis
      {LM}{\pro}    % linear models
      {NC}{\apl}    % numeric calculus
      {NIPDE}{\apl} % numeric integration of pde
      {PDE}{\apl}   % partial differential equations
      {RFA}{\ana}   % real and functional analysis
      {SP}{\pro}    % stochastic processes
      {ADS}{\apl}   % advanced dynamical systems
      {AP}{\pro}    % advanced probability
      {ATFAPDE}{\ana} % advanced topics in functional analysis and pdes
      {IEPDE}{\ana} % introduction to evolution pdes
      {INEPDE}{\ana} % introduction to non linear elliptic pdes
      {LTLD}{\pro} % limit theorems and large deviations
      {SC}{\pro}    % stochastic calculus
      {INLP}{\phy}  % Instabilities and nonlinear phenomena
      {MM}{\pro}    % Montecarlo methods
      {CO}{\apl}    % Continuous optimization
      {NMPDE}{\apl} % Numerical methods for pdes
      {SCO}{\pro}   % Stochastic control
      {JP}{\pro}    % Jump processes
      {ICT}{\ana}   % Introduction to control theory
  }{\col}%
}
\ExplSyntaxOff
%%%%%%%%%%%%%%%%%%%%%%%

\newcommand\FirstWord[1]{\@removedots#1:\@nil}%
\newcommand\@removedots{}%
\def\@removedots#1:#2\@nil{#1}%

\newcommand\changecolor[1]{
  \renewcommand{\col}{\refcol{#1}}
  \hypersetup{linkcolor = \col}
}

%% for references (hyperref)
\hypersetup{
  colorlinks = true,
  linkcolor = Blue4, % color of internal links (sections, pages, etc.) 
  filecolor = cyan, % color for URLs which open local files
  citecolor = green, %	color for bibliographical citations in text.     
  urlcolor = cyan, % color of URL links (mail, web) 
}
%%%%%%%%%%%%%%%%%%%%

%% Color box for important theorems
\def\sep{1mm}
\newtcolorbox{important}{colback=\col!5, colframe=\col, left=\sep, right=\sep, top=\sep, bottom=\sep}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%% Enumeration - commands %%%%%%%
\usepackage{enumitem} % for the command [leftmargin=1.15cm] in customized-label enumerations
\renewcommand{\labelenumi}{\arabic{enumi}.}
\renewcommand{\theenumi}{\thetheorem-\arabic{enumi}}

\renewcommand{\labelenumii}{\roman{enumii})}
\renewcommand{\theenumii}{\theenumi.\roman{enumii})}

\renewcommand{\labelenumiii}{\alph{enumiii})}
\renewcommand{\theenumiii}{\theenumii.\alph{enumiii})}


% for doing things like:
% 1. ..............
% 2. ..............
% 3, 4. ...........
% 5. ..............
\newcommand{\doubleitem}{%
  \begingroup
  \stepcounter{enumi}%
  \edef\tmp{\arabic{enumi}, }%
  \stepcounter{enumi}
  \edef\tmp{\endgroup\noexpand\item[\tmp\labelenumi]}%
  \tmp}

%% for proving two-way implications
% Usage:
% \begin{itemizeiff}
% First implication ...
% \item Second implication ...
% \end{itemizeiff}
\newlist{itemizeiff}{enumerate}{2}
\setlist[itemizeiff]{
  first=\item[{$\implies\!\!$)}],
  label={$\impliedby\!\!$)},
  leftmargin=1.2cm,
}

\newcommand*\ifftoright{\item[$\implies$:]}
\newcommand*\ifftoleft{\item[$\ \impliedby$:]}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%% Theorems, definitions,... environments %%%%%
\newtheoremstyle{math_theorems}
{\topsep}   % ABOVESPACE
{\topsep}   % BELOWSPACE
{}          % BODYFONT
{0pt}       % INDENT (empty value is the same as 0pt)
{\bfseries} % HEADFONT
{.}         % HEADPUNCT
{5pt plus 1pt minus 1pt} % HEADSPACE
{\color{\col}\thmname{#1}\thmnumber{\@ifnotempty{#1}{ }\@upn{#2}}%
  \thmnote{{ \bfseries\color{black}(#3)\index{#3}}}}          % CUSTOM-HEAD-SPEC

\theoremstyle{math_theorems}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{proposition}[theorem]{Proposition}
\newtheorem{corollary}[theorem]{Corollary}
\newtheorem{axiom}[theorem]{Axiom}
\newtheorem{definition}[theorem]{Definition}
\newtheorem{law}[theorem]{Law}
\newtheorem{principle}[theorem]{Principle}
\newtheorem{model}[theorem]{Model}

\newtheoremstyle{math_remarks}
{\topsep}   % ABOVESPACE
{\topsep}   % BELOWSPACE
{}          % BODYFONT
{0pt}       % INDENT (empty value is the same as 0pt)
{\itshape} % HEADFONT
{.}         % HEADPUNCT
{5pt plus 1pt minus 1pt} % HEADSPACE
{\color{\col}\thmname{#1}\thmnote{{ \color{black}(#3)}}}          % CUSTOM-HEAD-SPEC

\theoremstyle{math_remarks}
\newtheorem{remark}{Remark}

% \renewcommand*{\qed}{{\color{\col}\qed}}
% \renewcommand{\qedsymbol}{\textcolor{\col}{\qed}}
\newcommand*{\QED}[1][$\square$]{%
  \color{\col}
  \leavevmode\unskip\penalty9999 \hbox{}\nobreak\hfill
  \quad\hbox{#1}%
}

\makeatletter
\renewenvironment{proof}[1][\proofname]{
  \par%
  \pushQED{\QED}%
  \normalfont%
  \topsep5\p@\@plus\p@\@minus\p@\relax%
  \trivlist%
  \item\relax%
  {%
    \color{\col}%
    \itshape#1\@addpunct{.}}\hspace\labelsep\ignorespaces%
}{%
  % \color{\col!30}
  \popQED\endtrivlist\@endpefalse%
}
\makeatother

\newcommand\explanationname{Sketch of the proof} % could be made language-dependent
\newenvironment{sproof}%
{\renewcommand\proofname\explanationname%
  \begin{proof}}%
    {\end{proof}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%% Extra references %%%%%%%
\makeatletter
\if@cref@capitalise % if we pass the option 'capitalise' on cleveref package
  % \crefname{theorem}{\bfseries\color{\col} Theorem}{\bfseries\color{\col} Theorems}
  % \crefname{lemma}{\bfseries\color{\col} Lemma}{\bfseries\color{\col} Lemmas}
  % \crefname{proposition}{\bfseries\color{\col} Proposition}{\bfseries\color{\col} Propositions}
  % \crefname{corollary}{\bfseries\color{\col} Corollary}{\bfseries\color{\col} Corollary}
  % \crefname{axiom}{\bfseries\color{\col} Axiom}{\bfseries\color{\col} Axioms}
  % \crefname{definition}{\bfseries\color{\col} Definition}{\bfseries\color{\col} Definitions}
  % \crefname{law}{\bfseries\color{\col} Law}{\bfseries\color{\col} Laws}
  % \crefname{principle}{\bfseries\color{\col} Principle}{\bfseries\color{\col} Principles}
  % \crefname{enumi}{\textcolor{\col}{Item}}{\textcolor{\col}{Items}}
  % \else
  % \crefname{theorem}{\bfseries\color{\col} theorem}{\bfseries\color{\col} theorems}
  % \crefname{lemma}{\bfseries\color{\col} lemma}{\bfseries\color{\col} lemmas}
  % \crefname{proposition}{\bfseries\color{\col} proposition}{\bfseries\color{\col} propositions}
  % \crefname{corollary}{\bfseries\color{\col} corollary}{\bfseries\color{\col} corollary}
  % \crefname{axiom}{\bfseries\color{\col} axiom}{\bfseries\color{\col} axioms}
  % \crefname{definition}{\bfseries\color{\col} definition}{\bfseries\color{\col} definitions}
  % \crefname{law}{\bfseries\color{\col} law}{\bfseries\color{\col} laws}
  % \crefname{principle}{\bfseries\color{\col} principle}{\bfseries\color{\col} principles}
  % \crefname{enumi}{\textcolor{\col}{item}}{\textcolor{\col}{items}}
  \crefname{axiom}{Axiom}{Axioms}
\else
  \crefname{axiom}{axiom}{axioms}
\fi
\makeatother

\newcommand{\mref}[1]{\hypersetup{linkcolor = \refcol{#1}}\ref{#1}\hypersetup{linkcolor = \col}}         % custom ref
\newcommand{\mcref}[1]{\hypersetup{linkcolor = \refcol{#1}}\cref{#1}\hypersetup{linkcolor = \col}}       % custom cref
% \newcommand{\mnameref}[1]{\hypersetup{linkcolor = \refcol{#1}}\nameref{#1}\hypersetup{linkcolor = \col}} % custom nameref
\newcommand{\mnameref}[1]{\hypersetup{linkcolor = \refcol{#1}}\hyperref[{#1}]{\ref{#1} \nameref{#1}}\hypersetup{linkcolor = \col}} % custom nameref
% \newcommand{\mfullref}[1]{\hypersetup{linkcolor = \refcol{#1}}\hyperref[{#1}]{\ref{#1} \nameref{#1}}\hypersetup{linkcolor = \col}}
% \newcommand{\fullref}[1]{\hypersetup{linkcolor = \refcol{#1}}\hyperref[{#1}]{\autoref{#1} \nameref{#1}}\hypersetup{linkcolor = \col}}
% \crefformat{enumerate}{#2\textcolor{\col}{Item} #1#3}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%