I want to be able to customize my header/footer so far that I can change the rules that separate it from the document. Look for example at http://ww1.microchip.com/downloads/en/appnotes/00734b.pdf (second page and further), I want to be able to make header/footer rules like that.

Currently I'm using the fancyhdr package, but that's not necessary.
I already tried to add \hrule in the footer, and that's working, but I'd think there is a neater solution?

The code I used for that:

\renewcommand{\footrulewidth}{0pt}
\getfile\vskip2pt
\hrule width \hsize height 1pt\vskip1pt\hrule width \hsize height 2pt %
}
\fancyfoot[L]{
\hrule width \hsize height 2pt\vskip1pt\hrule width \hsize height 1pt %
\vskip2pt Some text.
}
\fancyfoot[C]{\vskip5pt Some text.}
\fancyfoot[R]{\vskip5pt Some text.}


To replace the rules of fancyhdr, you can redefine \headrule and \footrule including just the solution at your question Two rules directly under each other

\documentclass{article}

\usepackage{lastpage} % number of last page
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}

\renewcommand\footrule{\begin{minipage}{1\textwidth}
\hrule width \hsize height 2pt \kern 1mm \hrule width \hsize
\end{minipage}\par}%

\begin{minipage}{1\textwidth}
\hrule width \hsize \kern 1mm \hrule width \hsize height 2pt
\end{minipage}}%