\documentclass[12pt,a4paper]{book}
%\usepackage{amsmath,amssymb,amsthm,color}
\usepackage{coolthms}
\usepackage{enumitem}


\setenumerate[1]{leftmargin=*,labelindent=\parindent,label=(\alph*)}
\setenumerate[2]{leftmargin=*,labelindent=\parindent,label=(\roman*),%
ref=\theenumi~(\roman*)}

\definetheorem{thm}{Theorem}[chapter]


\begin{document}
\chapter{Linear Algebra}

\begin{thm}
Test theorem.
\begin{enumerate}
\item First point.
\begin{enumerate}
\item First sub-point.
\item\Label{testlabel} Second sub-point.
\end{enumerate}
\item Second point.
\end{enumerate}
\end{thm}

in \cref{testlabel}
\end{document}