\documentclass[a4paper,10pt]{article}

\input{common.tex}

\definecolor{light-gray}{gray}{0.75}

\begin{document}
\begin{figure}[ht]
\centering
\begin{tikzpicture}[x=1cm,y=1cm]

\draw[step=4mm,light-gray,line width=0.1pt] (0,0) grid (20.8,29.6);

\end{tikzpicture}
\end{figure}
\end{document}