header.tex

% Tikz lets us draw anything
\usepackage{tikz}
\usetikzlibrary{calc} % Coordinate calculation in tikz

% Allows drawing in the background
\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}

% Allows customizing the headings
\usepackage[explicit]{titlesec}

\newfontfamily\rsfamily{Roboto Slab}
\newfontfamily\sspfamily{Source Sans Pro}

% Define some colors
\definecolor{csyellow}{HTML}{FFE66D}
\definecolor{csteal}{HTML}{4ECDC4}
\definecolor{csred}{HTML}{FF6B6B}
\definecolor{csdarkgreen}{HTML}{1A535C}
\definecolor{csdark}{HTML}{0A1F22}
\definecolor{cslight}{HTML}{F7FFF7}

% Fancy chapter headings
\titleformat{\chapter}[display]
  {\normalfont\rsfamily\Huge\fontseries{m}}
  {}
  {\dimexpr-\oddsidemargin-\hoffset-1in}
  {%
    \begin{tikzpicture}[]
      \draw (0,0) +(0,0) node[anchor=west](head){\thechapter{}.~#1};
      \begin{pgfonlayer}{bg}
        \fill [csyellow] ($(head.south west)+(-0.1,0.05)$) rectangle ($(head.south east)+(-0.4,0.37)$);
      \end{pgfonlayer}
    \end{tikzpicture}%
  }

% Less fancy headings (but with custom font)
\titleformat{\section}
  {\normalfont\rsfamily\Large\fontseries{m}}
  {\thesection.}{1em}{#1}
\titleformat{\subsection}
  {\normalfont\rsfamily\large\fontseries{m}}
  {\thesubsection.}{1em}{#1}