Fix page style for first ToC page

hhuthesis.cls

 \NeedsTeXFormat{LaTeX2e}
-\ProvidesClass{hhuthesis}[2021/06/28 HHU Thesis LaTeX document class v1.20]
+\ProvidesClass{hhuthesis}[2023/08/24 HHU Thesis LaTeX document class v1.21]
 
 \DeclareOption*{\PassOptionsToClass{\CurrentOption}{article}} % Options fallback.
 \ProcessOptions\relax
@@ -121,6 +121,8 @@
 \RequirePackage[scaled=0.95]{helvet}
 
 \pagestyle{headings}
+%% First page of ToC somehow forces page style to plain. This fixes it.
+\AtBeginDocument{\addtocontents{toc}{\protect\thispagestyle{headings}}}
 
 \setlength{\textwidth}{15cm}