Use Geometry Package for custom page margin settings after header

Question

I want to use geometry package for custom page margin setting after the header. Currently my documents looks like

I want to use geometry package to expand EDUCATION section so it should look like as follows

I've generated the above image using following code

\documentclass{article}
\usepackage{tikz}
\usepackage{xcolor}
\usepackage[parfill]{parskip} % Remove paragraph indentation
\usepackage{array} % Required for boldface (\bf and \bfseries) tabular columns
\usepackage{ifthen} % Required for ifthenelse statements
\pagenumbering{gobble} 
\usepackage[left=0.75in,top=0.6in,right=0.75in,bottom=0.6in]{geometry} 

\definecolor{darkgray}{HTML}{394456}
\definecolor{blue}{HTML}{5A7294}
\definecolor{white}{RGB}{255,255,255}

\def\sectionlineskip{\medskip}
\def\sectionskip{\medskip} % The space after the heading section

% Defines the rSection environment for the large sections within the CV
\newenvironment{rSection}[1]{ % 1 input argument - section name
  \sectionskip
  \MakeUppercase{\bf #1} % Section title
  \sectionlineskip
  \hrule % Horizontal line
  \begin{list}{}{ % List for each individual item in the section
    \setlength{\leftmargin}{1.5em} % Margin within the section
  }
  \item[]
}{
  \end{list}
}



\begin{document}


\begin{rSection}{Education}

{\bf University} \hfill {\em Start - End} 
\\ Degree\hfill { Overall CGPA: 4}

\end{rSection}

\end{document

Now if I don't use the geometry package my code for the first image would be

\documentclass{article}
\usepackage{tikz}
\usepackage{xcolor}
\usepackage[parfill]{parskip} % Remove paragraph indentation
\usepackage{array} % Required for boldface (\bf and \bfseries) tabular columns
\usepackage{ifthen} % Required for ifthenelse statements
\pagenumbering{gobble} 


\definecolor{darkgray}{HTML}{394456}
\definecolor{blue}{HTML}{5A7294}
\definecolor{white}{RGB}{255,255,255}

\def\sectionlineskip{\medskip}
\def\sectionskip{\medskip} % The space after the heading section

% Defines the rSection environment for the large sections within the CV
\newenvironment{rSection}[1]{ % 1 input argument - section name
  \sectionskip
  \MakeUppercase{\bf #1} % Section title
  \sectionlineskip
  \hrule % Horizontal line
  \begin{list}{}{ % List for each individual item in the section
    \setlength{\leftmargin}{1.5em} % Margin within the section
  }
  \item[]
}{
  \end{list}
}

\begin{document}


\begin{tikzpicture}[remember picture,overlay]
\node [rectangle, fill=darkgray, anchor=north, minimum width=\paperwidth, minimum height=3cm] (box) at (current page.north){};


\node [text=white,font=\fontsize{45pt}{65pt}\bfseries\sffamily,anchor=north
west] (name) at ([xshift=2em,yshift=-2em]box.north west) {%
John Doe
};
\node [text=blue!20,font=\fontsize{15pt}{15pt}\bfseries\sffamily,anchor=north
west] (name) at ([xshift=0.5ex]name.south west) {%
Job title
};

\node[anchor=east] at ([xshift=-2em]box.east)
{\includegraphics[height=2cm]{example-image-duck}};
\end{tikzpicture}


\begin{rSection}{Education}

{\bf University} \hfill {\em Start - End} 
\\ Degree\hfill { Overall CGPA: 4}

\end{rSection}

\end{document}

How can I get header from first image and education section with custom margin settings using geometry package as demonstrated in second image?

I need to use margin settings only for the part after my header.

Answer 1

Just include the tikzpicture after \begin{document} and increase the top margin to leave enough space (the TikZ picture takes no space because it uses the overlay option—it appears as a zero-size box for TeX at the point where it occurs).

Use \newcommand rather than \def and \textbf rather than \bf. \emph is easier to use than \em in general because it automatically provides italic correction.

I removed possible spurious spaces and added \par\nopagebreak after your section title to ensure the following \medskip isn't a legal breakpoint.

The font sizes your code requests are not available unless you change your font setup. I added \usepackage{lmodern}, this makes them available. I also replaced Start - End with Start -- End (em dash: longer than a hyphen).

\documentclass{article}
\usepackage{lmodern}
\usepackage{tikz}
\usepackage[parfill]{parskip} % Remove paragraph indentation
\usepackage[left=0.75in,top=3cm,right=0.75in,bottom=0.6in]{geometry}

\definecolor{darkgray}{HTML}{394456}
\definecolor{blue}{HTML}{5A7294}
\definecolor{white}{RGB}{255,255,255}

\newcommand{\sectionlineskip}{\medskip}
\newcommand{\sectionskip}{\medskip}

% Defines the rSection environment for the large sections within the CV
\newenvironment{rSection}[1]{% 1 input argument - section name
  \sectionskip
  \MakeUppercase{\bf #1}% Section title
  \par\nopagebreak
  \sectionlineskip
  \hrule % Horizontal line
  \begin{list}{}{% List for each individual item in the section
    \setlength{\leftmargin}{1.5em} % Margin within the section
  }%
  \item[]
}{%
  \end{list}
}

\pagestyle{empty}

\begin{document}

\begin{tikzpicture}[remember picture, overlay]
\node[rectangle, fill=darkgray, anchor=north, minimum width=\paperwidth,
      minimum height=3cm] (box) at (current page.north) {};

\node[text=white,font=\fontsize{45pt}{65pt}\bfseries\sffamily,
      anchor=north west] (name) at ([xshift=2em,yshift=-2em]box.north west)
  {John Doe};

\node[text=blue!20, font=\fontsize{15pt}{15pt}\bfseries\sffamily,
      anchor=north west] (name) at ([xshift=0.5ex]name.south west)
  {Job title};

\node[anchor=east] at ([xshift=-2em]box.east)
  {\includegraphics[height=2cm]{example-image-duck}};
\end{tikzpicture}

\begin{rSection}{Education}
\textbf{University}\hfill \emph{Start -- End}\\
Degree\hfill Overall CGPA: 4
\end{rSection}

\end{document}