revert theorem usage to amsthm style

elegantbook-en.tex

@@ -175,6 +175,7 @@ BTW, we provide Certificate of Donation for those who donated, tippers please le
   \includegraphics[width=0.7\textwidth]{cert.pdf}
 \end{figure}
 
+\part{Hello world}
 \chapter{ElegantBook Settings}
 
 This template is based on the Standard \LaTeX{} book class, so the options of book class work as well (Note that the option of papersize has no effect due to \lstinline{device} option). The default encoding is UTF-8 while \TeX{} Live is recommended. The test environment is Win10 + \TeX{} Live 2019, either \hologo{pdfLaTeX} or \lstinline{XeLaTeX} works fine. \lstinline{XeLaTeX} is preferred for Chinese articles.
@@ -611,7 +612,7 @@ The reason is that the template redefines font for math so that no new math font
 
 We will define the integral of a measurable function in three steps. First, we define the integral of a nonnegative simple function. Let $E$ be the measurable set in $\mathcal{R}^N$.
 
-\begin{definition}{Left Coset}{}
+\begin{definition}[Left Coset]
 Let $H$ be a subgroup of a group~$G$.  A \emph{left coset} of $H$ in $G$ is a subset of $G$ that is of the form $xH$, where $x \in G$ and $xH = \{ xh : h \in H \}$. Similarly a \emph{right coset} of $H$ in $G$ is a subset of $G$ that is of the form $Hx$, where $Hx = \{ hx : h \in H \}$ $\hbar$
 \end{definition}
 
@@ -621,10 +622,11 @@ Note that a subgroup~$H$ of a group $G$ is itself a left coset of $H$ in $G$.
 
 \lipsum[2]
 
-\begin{theorem}{Lagrange's Theorem}{}
+\begin{theorem}[Lagrange's Theorem] \label{thm:lg}
 Let $G$ be a finite group, and let $H$ be a subgroup of $G$.  Then the order of $H$ divides the order of $G$.
 \end{theorem}
 
+\ref{thm:lg}
 \lipsum[3]