如何改变定理环境主体中的字体形状？

Question

\documentclass{article}
\usepackage{amsmath,amssymb}
\newtheorem{defn}{Definition}
\newtheorem{pro}{Problem}
\newtheorem{thm}{Theorem}
\begin{document}
\begin{defn}
This is the first definition
\end{defn}
\begin{thm}
This is the first theorem
\end{thm}
\end{document}

当我得到输出时，定义、定理的主体是斜体。我能以正常字母形式获得这些环境吗？

Answer 1

无需任何与定理相关的包（使用etoolbox修补 LaTeX 内核):

\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@begintheorem}{\itshape}{}{}{}
\patchcmd{\@opargbegintheorem}{\itshape}{}{}{}
\makeatother
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}

和amsthm：

\documentclass{article}
\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}

和ntheorem：

\documentclass{article}
\usepackage{ntheorem}
\theorembodyfont{\normalfont}
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}

加上amsthm​thmtools前端（也可以与一起使用ntheorem）：

\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheoremstyle[bodyfont=\normalfont]{normalbody}
\declaretheorem[style=normalbody,name=Problem]{pro}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}