Numbering with greek numeral of exercises with xsim package

Question

I would like make xsim numbering my exercises with greek numerals. For example instead of

• Θέμα 1
• Θέμα 2 ...

i would like

• Θέμα Α
• Θέμα Β ... etc

How can i modify or set the counter?

I have tried \Alph{counter}

\documentclass[a4paper, 11pt]{memoir}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
% XeLaTeX
\usepackage{mathspec}
\usepackage{xgreek}
\setmainfont{Google Sans}
\setmathfont(Greek,Latin,Digits)[Uppercase=Regular, Lowercase=Regular, Arabic=Regular]{Google Sans}
\usepackage[clear-aux]{xsim}

\DeclareExerciseEnvironmentTemplate{default2}{%
    \GetExerciseHeadingF{\subsection*}%
    {%
        \XSIMmixedcase{\GetExerciseName}\nobreakspace%
        \GetExerciseProperty{counter}
        \IfInsideSolutionF{ \GetExercisePropertyT{subtitle}{ {\normalfont\itshape\PropertyValue} } }%
        }
    }
{\par}
%
\DeclareExerciseType{myquestion}{
    exercise-env = question ,
    solution-env = qsolution ,
    exercise-name = Θέμα ,
    solution-name = Ενδεικτική λύση ,
    exercise-template = default2 ,
    solution-template = default2
    }
%
\DeclareExerciseCollection{test}

\begin{document}
    \activatecollection{test}
        \xsimsetup{ collect }
        \begin{question}
            q.1
        \end{question}
        \begin{qsolution}
            solution 1
        \end{qsolution}
        \begin{question}
            q.2
        \end{question}
        \begin{qsolution}
            solution 2
        \end{qsolution}
    \deactivatecollection{test}
    %
    \printcollection{test}
    \printallsolutions
\end{document}