documentclass[twoside]{scrbook}
usepackage[letterpaper, left=1.5cm, right=1.5cm, bindingoffset=1.5cm]{geometry}
usepackage[utf8]{inputenc}
usepackage{fancyhdr}
usepackage{titlesec}
usepackage{tikz}
usepackage{lipsum}
usepackage{etoolbox}
patchcmd{chapter}{thispagestyle{plain}}{thispagestyle{fancy}}{}{}

definecolor{gmitblue}{RGB}{93,138,168}
definecolor{midnightgreen}{rgb}{0.0, 0.29, 0.33}

usetikzlibrary{calc}
renewcommand{headrulewidth}{0pt}

pagestyle{fancy}
fancyhf{}
fancyhead[CO]{%
begin{tikzpicture}[overlay, remember picture]%
fill[midnightgreen] ($(current page.north west)+(0.25in,-0.35in)$) rectangle ($(current page.north west)+(0.65in,-0.75in)$);
    fill[midnightgreen] ($(current page.north west)+(2.5in,-0.35in)$) rectangle ($(current page.north east)+(-0.6in,-0.75in)$);
node[anchor=north west, text=white, font=largebfseries, minimum size=1in, inner xsep=5mm] at ($(current page.north west)+(-0.1in,-0.05in)$){thepage};
node[anchor=north west, text=white, font=largebfseries, minimum size=1in] at ($(current page.north west)+(2.6in,-0.075in)$) {rightmark};
end{tikzpicture}
}
fancyhead[CE]{%
begin{tikzpicture}[overlay, remember picture]%
fill[midnightgreen] ($(current page.north west)+(0.6in,-0.35in)$) rectangle ($(current page.north east)+(-2.25in,-0.75in)$);
fill[midnightgreen] ($(current page.north east)+(-0.65in,-0.35in)$) rectangle ($(current page.north east)+(-0.25in,-0.75in)$);
node[anchor=north west, text=white, font=largebfseries, minimum size=1in, inner xsep=5mm] at ($(current page.north east)+(-1in,-0.05in)$){thepage};
node[anchor=north west, text=white, font=largebfseries, minimum size=1in] at ($(current page.north west)+(0.65in,-0.075in)$) {leftmark};
end{tikzpicture}
}
setlength{headheight}{12pt}

title{A reasonably long title}
date{today}
author{The author}

begin{document}
maketitle
chapter[Hola Mundo]{Hola Mundo}
lipsum[1-9]
section{La prueba na' ma'}

chapter[Hola de Nuevo]{Hola Mundo... de Nuevo}
lipsum[10-19]
end{document}