Adjust pagestyle when switching to `\mainmatter`

hhuthesis.cls

 \NeedsTeXFormat{LaTeX2e}
-\ProvidesClass{hhuthesis}[2023/10/23 HHU Thesis LaTeX document class v1.25]
+\ProvidesClass{hhuthesis}[2023/10/23 HHU Thesis LaTeX document class v1.26]
 
 \DeclareOption*{\PassOptionsToClass{\CurrentOption}{article}} % Options fallback.
 \ProcessOptions\relax
@@ -154,7 +154,10 @@
 }
 
 \newcommand{\mainmatter}{
+  \clearpage
+  \pagestyle{plain}
   \cleardoublepage
+  \pagestyle{headings}
   \pagenumbering{arabic}
 }