I would like to set the math font as Garamond-Math (here) except that greek letters should come from a different font (for an illustration, computer modern in the example below.). But it seems declaring the new font does not work.
How to achieve this?
\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage[math-style=ISO, bold-style=ISO]{unicode-math}
\setmathfont{Garamond-Math.otf}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\DeclareSymbolFont{gs}{OML}{cmm}{m}{it}
\DeclareMathSymbol{\alpha}{\mathalpha}{gs}{11}
\DeclareMathSymbol{\beta}{\mathalpha}{gs}{12}
\DeclareMathSymbol{\gamma}{\mathalpha}{gs}{13}
\begin{document}
\[2a\alpha=\theta\]
\end{document}