How to put bracket around the number of a theorem?

Question

What I want is like this:

(1.1) Theorem. blabla...

I have used ntheorem package to interchange the number of a theorem and its name. Then, how to put the bracket around the number?

\documentclass{book}
 \usepackage{amsmath}
 \usepackage[amsmath,thmmarks]{ntheorem}
  \theoremstyle{change}
  \theorembodyfont{\itshape}
   \newtheorem{thm}{Theorem}[section]

\begin{document}
 \chapter{A Chapter}
 \section{A section}
   \begin{thm}
     A theorem.
   \end{thm}
\end{document}

Answer 1

Define a new theorem style mychange

\makeatletter 
 \newtheoremstyle{mychange}%
  {\item[\hskip\labelsep \theorem@headerfont (##2)\ ##1\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont (##2)\ ##1\ (##3)\theorem@separator]}
\makeatother

and use

\theoremstyle{mychange}

MWE

\documentclass{book}
 \usepackage{amsmath}
 \usepackage[amsmath,thmmarks]{ntheorem}

\makeatletter 
 \newtheoremstyle{mychange}%
  {\item[\hskip\labelsep \theorem@headerfont (##2)\ ##1\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont (##2)\ ##1\ (##3)\theorem@separator]}
\makeatother

  \theoremstyle{mychange}
  \theorembodyfont{\itshape}
   \newtheorem{thm}{Theorem}[section]

\begin{document}
 \chapter{A Chapter}
 \section{A section}
   \begin{thm}
     A theorem.
   \end{thm}
\end{document}