# How do I change counter to Alph format of a counter in tcolorbox

I am trying to implement the idea of a bigthm counter---so make the numbering Alph format--- in a tcolorbox theoremlike environment. With the amsthm package, what I needed to do was

\newtheorem{bigthm}{Theorem}
\renewcommand{\thebigthm}{\Alph{bigthm}}


the same thing doesn't work with a tcolorbox theorem: the following two lines

\newtcbtheorem{bigtcthm}{Theorem}{}{}
\renewcommand{\thebigtcthm}{\Alph{bigtcthm}}


give me the error

./bigthm_counter.tex:10: LaTeX Error: \thebigtcthm undefined.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
...

l.10 \renewcommand{\thebigtcthm}
{\Alph{bigtcthm}}


Here is a minimal example comparing the two theoremlike environments, the one from amsthm and the other from tcolorbox

\documentclass{article}

\newtheorem{bigthm}{Theorem}
\renewcommand{\thebigthm}{\Alph{bigthm}}

\usepackage{tcolorbox}
\tcbuselibrary{theorems}

\newtcbtheorem{bigtcthm}{Theorem}{}{}
\renewcommand{\thebigtcthm}{\Alph{bigtcthm}}

\begin{document}
\begin{bigthm}
$1=1$
\end{bigthm}

\begin{bigtcthm}{}{}
$1=1$
\end{bigtcthm}
\end{document}


I now see in the tcolorbox manual that adding number format=\Alph as an option in the bigtcthm environment definition works.