# How to change the style of a theorem at LyX?

As a continue to this question - Why the amstools doesn't work here? - I think to ask it from a different view:

I'm using this at Lyx and I want to change the style of the theorems like here: How to produce theorems in which the color of heading and body of the theorem my differ?

The first two answers doesn't work at all, and make problems, like here: Why the amstools doesn't work here? (same question above)

So my questions is: There is a way to change the style of theorems and things like that without creating new ones? Because, as I saw at the answers - the just redesign the style and don't create a new object.
(remember that I'm using LyX...)

Thank you!!

% Preview source code
%% LyX 2.3.0 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[a4paper,english,hebrew]{scrartcl}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{fontspec}
\usepackage{fancyhdr}
\pagestyle{fancy}
\setlength{\parindent}{0bp}
\usepackage[unicode=true,pdfusetitle,
bookmarks=true,bookmarksnumbered=false,bookmarksopen=false,
{hyperref}

\makeatletter

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands.
\pdfpageheight\paperheight
\pdfpagewidth\paperwidth

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
\theoremstyle{plain}
\ifx\thechapter\undefined
\newtheorem{thm}{\protect\theoremname}
\else
\newtheorem{thm}{\protect\theoremname}[chapter]
\fi

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\newfontfamily\hebrewfont[Script=Hebrew]{David CLM}
\newfontfamily\hebrewfonttt[Script=Hebrew]{Miriam Mono CLM}
\newfontfamily\hebrewfontsf[Script=Hebrew]{Yehuda CLM}
\AtBeginDocument{
\renewcommand\footnoterule{%
\kern -3pt
\hbox to \textwidth{\hfill\vrule height 0.4pt width .4\textwidth}
\kern 2.6pt
}}
% declare a new theorem style
\newtheoremstyle{mystyle}%
{3pt}% Space above
{3pt}% Space below
{\itshape\color{red}}% Body font
{}% Indent amount
{\bfseries\color{blue}}% Theorem head font
{.}% Punctuation after theorem head
{.5em}% Space after theorem head
{}% Theorem head spec (can be left empty, meaning ‘normal’)

% use the new theorem style
\theoremstyle{mystyle}
\newtheorem{thm}{Theorem}

\makeatother

\usepackage{polyglossia}
\setdefaultlanguage{hebrew}
\setotherlanguage{english}