Is possible to decrease the text margin of a theorem automatically?

Question

I am writing a LaTeX document where I formatting a mathstatement environment as into the following code.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \documentclass[10pt]{report} 
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\usepackage{geometry}
\usepackage{layout}

\usepackage{parskip} 
\usepackage[hang,flushmargin]{footmisc} 
\usepackage{fontsize} 
\usepackage{soul} 
\usepackage{titlesec} 
\usepackage[T1]{fontenc} 
\usepackage{fontspec} 

\usepackage{graphicx}
\usepackage{pdfpages}
\usepackage{caption} 
\usepackage{float} 
\usepackage{hyperref} 

\usepackage{enumitem} 
\usepackage{moreenum} 

\usepackage{amsmath} 
\usepackage{amsthm}

\usepackage{amssymb} 
\usepackage{stmaryrd} 

\usepackage{amsfonts} 
\usepackage{mathrsfs} 
\usepackage{yfonts}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\geometry{paperheight=29.7cm,paperwidth=21cm,textwidth=17cm,textheight=25cm}

\setlength{\parindent}{0.25cm}

\linespread{1.5}

\setlength{\parskip}{1pt}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newtheorem{definition}[equation]{Definizione}

\newtheorem{axiom}[equation]{Assioma}

\newtheorem{lemma}[equation]{Lemma}

\newtheorem{proposition}[equation]{Proposizione}

\newtheorem{corollary}[equation]{Corollario}

\newtheorem{theorem}[equation]{Teorema}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\usepackage{lipsum}

\begin{document}

    \lipsum[1]

    \begin{theorem}[My Theorem]
        \leftskip=1cm
        \rightskip=1cm
        \normalfont
        %\setmainfont{*Actually I would like write the theorem into a particular upright font: e.g. Foothlight MT Regular, Cormorant Garamont, gfsartemisia, etc...*}
        \quad\\
        This is a Theorem formatted in my personal style.
    \end{theorem}

    \lipsum[2]
\end{document}

So to automatize the process I tried to use the commands \newtheoremstyle and \theoremstyle but I was not able to indicathe the value of \leftskip and \rightskip and I was not also able to indicate that the text must be into a new line and not into the same of the title.

So I would like if it is possibile to automatize the process: could someone help me, please?

N.B.

To follow the way I tried to use \newtheoremstyle: I point out that the text is actually into a new line but the space between the title and the text is really big!

\newtheoremstyle{enunciato}% The name used to refer to the new style
  {\topsep}% The vertical space above the head list, a rubber lenght (default \topsep)
  {\topsep}% The vertical space below the head list, a rubber lenght (default \topsep)
  {%A declaration of the font and other aspects of the style to use for the text in the body of the list (default \normalfont)
   \setmainfont{CormorantGaramontItalic}[
                                Path = Fonts/CormorantGaramont/ ,
                                Extension = .ttf ,
                                UprightFont = CormorantGaramond-Italic
                              ]
  }% name of font to use in the body of the theorem
  { }% The extra indentation of the first line of the list, a non-rubber lenght (default is no extra indent)
  {%A declaration of the font and other aspects of the style to use for the text in the head of the list (default \normalfont)
   \setmainfont{CormorantGaramontBold}[
                                Path = Fonts/CormorantGaramont/ ,
                                Extension = .ttf ,
                                UprightFont = CormorantGaramond-Bold
                              ]}% name of head font
  { }% The text (typically punctuation) to be inserted after the head text, including any note text.
  {\newline}% The horizontal space to be inserted after the head text and "punctuation", a rubber lenght. It cannot be completely empty. As two very specital cases it can contain either a single space charactere to indicate jus a normal interword space is required or, more suprisingly, just the command \newline to indicate that a new line should be started for the body of the list.
  { }% A non-empty value for this aargument enables a complete specification of the setting of the bead itself to be supplied; an empty value means that the layout of the "plain" theorem style is used. See below for further details.

Answer 1

Yes, you can.

But, for your own sanity, never change \setmainfont inside a document.

Of course the theorem header and the body font should be the same as the document's. Unless you want a Frankenstein document.

\documentclass[10pt]{report}
\usepackage{amsthm}
\usepackage{etoolbox}
\usepackage{lipsum}
%%% the needed changes https://tex.stackexchange.com/a/67251/4427
\makeatletter
\patchcmd{@thm}
  {\trivlist}
  {\list{}{\leftmargin=\theoremmargin\rightmargin=\theoremmargin\parsep=0pt}}
  {}{}
\patchcmd{@endtheorem}{\endtrivlist}{\endlist}{}{}
\makeatother
\newcommand{\theoremmargin}{1cm}
\newtheoremstyle{enunciato}
  {\topsep}
  {\topsep}
  {\normalfont}
  {}
  {\bfseries}
  {}
  {\newline}
  {}
\theoremstyle{enunciato}
\newtheorem{definition}[equation]{Definizione}
\newtheorem{axiom}[equation]{Assioma}
\newtheorem{lemma}[equation]{Lemma}
\newtheorem{proposition}[equation]{Proposizione}
\newtheorem{corollary}[equation]{Corollario}
\newtheorem{theorem}[equation]{Teorema}
\begin{document}
\lipsum[1]
\begin{theorem}[My Theorem]
This is a Theorem formatted in my personal style.
\lipsum[3][1-2]
\end{theorem}
\lipsum[2]
\end{document}

If you also want to modify proof along the same line,

\documentclass[10pt]{report}
\usepackage{amsthm}
\usepackage{etoolbox}
\usepackage{lipsum}
%%% the needed changes
\makeatletter
\patchcmd{@thm}
  {\trivlist}
  {\list{}{\setup@theorem}}
  {}{}
\patchcmd{@endtheorem}{\endtrivlist}{\endlist}{}{}
\makeatletter
\renewenvironment{proof}[1][\proofname]{\par
  \pushQED{\qed}%
  \normalfont \topsep6\p@@plus6\p@\relax
  \list{}{\setup@theorem}%
  \item[]\textit{#1}\par\nopagebreak\noindent\ignorespaces
}{%
  \popQED\endtrivlist@endpefalse
}
\newcommand{\setup@theorem}{%
  \leftmargin=\theoremmargin
  \rightmargin=\theoremmargin
  \parsep=0pt
  \listparindent=\parindent
}
\makeatother
\newcommand{\theoremmargin}{0.5cm}
\newtheoremstyle{enunciato}
  {\topsep}
  {\topsep}
  {\normalfont}
  {}
  {\bfseries}
  {}
  {\newline}
  {}
\theoremstyle{enunciato}
\newtheorem{definition}[equation]{Definizione}
\newtheorem{axiom}[equation]{Assioma}
\newtheorem{lemma}[equation]{Lemma}
\newtheorem{proposition}[equation]{Proposizione}
\newtheorem{corollary}[equation]{Corollario}
\newtheorem{theorem}[equation]{Teorema}
\begin{document}
\lipsum[1]
\begin{theorem}[My Theorem]
This is a Theorem formatted in my personal style.
\lipsum[3][1-2]
\end{theorem}
\begin{proof}
\lipsum[4][1-2]
\lipsum[5][1-2]
\end{proof}
\lipsum[2]
\end{document}

I find it ugly, but it's your document.