Update on Overleaf.

main.tex

@@ -4,7 +4,13 @@
 % ===== PAGE LAYOUT
 \usepackage[a4paper,width=150mm,top=25mm,bottom=25mm]{geometry}
 \usepackage[utf8]{inputenc}
-\usepackage{fancyhdr} 
+\usepackage{fancyhdr}
+\pagestyle{fancy}
+\renewcommand{\headrulewidth}{2pt}
+\fancyhf[lh,rh,ch]{}
+\fancyhf[leh]{\leftmark}
+\fancyhf[roh]{\rightmark}
+
 \usepackage{titlesec}
 \newcommand{\sectionbreak}{\clearpage}