I want to use \Alph{mycounter} to have my theorems labelled by capital roman letters and a short search led me to conclude that all I need to write is
\newcounter{mycounter}
\Alph{mycounter}
\theoremstyle{theorem}
\newtheorem{mainthm}[mycounter]{Theorem}
Upon compilation of
\begin{mainthm}
Something.
\end{mainthm}
\begin{mainthm}
Something else.
\end{mainthm}
I get "Theorem 1. Something." and "Theorem 2. Something else.". Do I need to fix something in my syntax? I also tried removing some possibly conflicting packages like amsthm and cleveref, to no avail.