For LaTeX 2025 the cls file should be updated as follow

[Ducasse]

going from

%%%
%%% Fonts & paragraph typography
%%%
\RequirePackage{fontspec,gentiumbook,opensans,FiraMono}

\setmainfont{Gentium Book Plus}

to

\RequirePackage{fontspec,opensans,FiraMono}

\setmainfont{GentiumBookPlus}
\setsansfont{Open Sans}[
  BoldFont = {* Bold},
  Scale=MatchLowercase,
  Numbers=OldStyle
]
\setmonofont{Fira Mono}[
  Scale=MatchLowercase,
  HyphenChar={-} % enable hyphenation (disabled by default in mono fonts)
  % caveat: LuaLaTeX doesn't support changing the actual character
  % alternative solution: \usepackage[htt]{hyphenat}
]

[Ducasse]

the following worked

\documentclass{article}
\usepackage{fontspec}
\usepackage{FiraMono}
\setmainfont{GentiumBookPlus} 

\setsansfont{Open Sans}[
  BoldFont = {* Bold},
  Scale=MatchLowercase,
  Numbers=OldStyle
]
\newcommand\HW{Hello World.}
\begin{document}
{\HW} \textsc{\HW} \textbf{\HW} \textit{\HW} \textbf{\textit{\HW}}

\textsf{\HW}
\textbf{\textsf{\HW}}
\texttt{\HW}

\end{document}

But I could not make sba works.