Lyx Making Theorems Not Italic Globally

Question

Before I used Lyx, I used \declaretheoremstyle to make the bodyfont into normal in theorems. But in Lyx, I'm using Theorems(AMS) and Theorems(Numbered by Chapters) Module and cannot find a way to make the bodyfont into normal(not italic). I've tried \theoremstyle{remark} in the preamble but it didn't work.

Currently, I'm just using \textup{} in every theorems but I just wish I could handle it in a global sense. The modules I'm using seem to add following commands:

\theoremstyle{plain}
\ifx\thechapter\undefined
\newtheorem{thm}{\protect\theoremname}
\else
\newtheorem{thm}{\protect\theoremname}[chapter]
\fi

It would be greatly appreciated if I can handle this easily! Thanks.

[Added] Here's the source code of what I'm dealing with.

% Preview source code
%% LyX 2.1.3 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[oneside,english]{scrbook}
\usepackage{mathptmx}
\usepackage[T1]{fontenc}
\usepackage{geometry}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage{bm}
\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amssymb}
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
\theoremstyle{plain}
\ifx\thechapter\undefined
\newtheorem{thm}{\protect\theoremname}
\else
\newtheorem{thm}{\protect\theoremname}[chapter]
\fi
\@ifundefined{date}{}{\date{}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\usepackage{indentfirst}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{thmtools}
\setlength{\parskip}{0.5mm}
\setlength{\parindent}{4.5mm}

\makeatother

\usepackage{babel}
\providecommand{\theoremname}{Theorem}
\begin{document}
\chapter{Euclidean Space}
\section{Tangent Space and Vector Field} 

\begin{thm}
\textup{Let}\end{thm}
\begin{proof}
$v_i (p)$
\end{proof}

\end{document}

Answer 1

You can easily get what you want using the ntheorem package instead of amsthm. One advantage is the placement of end-of-proof symbol will be fully automatic, even for proofs ending in a display equation:

    \documentclass[oneside,english]{scrbook}
    \usepackage{mathptmx}
    \usepackage[T1]{fontenc}
    \usepackage{geometry}
    \setcounter{secnumdepth}{3}
    \setcounter{tocdepth}{3}
    \usepackage{bm}
    \usepackage{amsmath}
    %\usepackage{amsthm}
    \usepackage[thmmarks, amsmath, thref]{ntheorem}
    \usepackage{amssymb}
    \makeatletter
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
    \theoremstyle{plain}
    \theoremheaderfont{\bfseries}
    \theorembodyfont{\upshape}
    \theoremseparator{.}
    \ifx\thechapter\undefined
    \newtheorem{thm}{\protect\theoremname}
    \else
    \newtheorem{thm}{\protect\theoremname}[chapter]
    \fi
    \theoremstyle{nonumberplain}
    \theoremheaderfont{\itshape}
    \theoremsymbol{\ensuremath{\square}}
    \newtheorem{proof}{Proof}
    \@ifundefined{date}{}{\date{}}
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
    \usepackage{indentfirst}
    \usepackage{thmtools}
    \setlength{\parskip}{0.5mm}
    \setlength{\parindent}{4.5mm}

    \makeatother

    \usepackage{babel}
    \providecommand{\theoremname}{Theorem}

    \begin{document}
    \chapter{Euclidean Space}
    \section{Tangent Space and Vector Field}

    \begin{thm}
    \textup{Let}\end{thm}
    \begin{proof}Since $ \omega $ is an exact form, $ ω=\mathrm d\mkern1mu f$, we have: 
\[ \int_{\gamma}\omega = \int_{\gamma}\mathrm d\mkern1mu f = f(\gamma(b)-f(\gamma(a). \]%
    \end{proof}

    \end{document}