LaTeX doesn't apply margins to enumerate/itemize within theorems, provide an option to do so.

[jdutant]

In LaTeX/PDF output, Enumerate and Itemize environment reset paragraphs margins (incl \leftskip, \rightskip). Hence setting left or right margins on theorems using \leftskip and \rightskip doesn't affect lists within theorems. If a theorem isn't a different font (e.g. remark) it may be look to the reader as if the list isn't in the theorem.

Solution: provide an option for applying margin to lists. For reference here's a LaTeX MWE using enumitem:

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumitem}
\usepackage{etoolbox}

% === Margin flag ===
\newif\iftheoremmargins
\theoremmarginsfalse

% === Command to set margins + flag ===
\newcommand{\settheoremmargins}[2]{%
  \theoremmarginstrue%
  \addtolength{\leftskip}{#1}%
  \addtolength{\rightskip}{#2}%
}

% === Redefine itemize/enumerate based on flag ===
\let\origitemize\itemize
\let\origenumerate\enumerate

\renewenvironment{itemize}
  {\iftheoremmargins
     \origitemize[left=2em]
   \else
     \origitemize
   \fi}
  {\endlist}

\renewenvironment{enumerate}
  {\iftheoremmargins
     \origenumerate[left=2em]
   \else
     \origenumerate
   \fi}
  {\endlist}

% === Define theorem style using the new macro ===
\newtheoremstyle{margintheorem}
  {3pt}{3pt}
  {\settheoremmargins{2em}{2em}} % Body font: sets skips and flag
  {}{\bfseries}{.}{.5em}
  {\thmname{#1}~\thmnumber{#2}\thmnote{ \normalfont(#3)}}

\theoremstyle{margintheorem}
\newtheorem{theorem}{Theorem}

% Optional: a plain style for comparison
\newtheoremstyle{plainstyle}
  {3pt}{3pt}{\itshape}{}{\bfseries}{.}{.5em}
  {\theoremmarginsfalse\thmname{#1}~\thmnumber{#2}\thmnote{ \normalfont(#3)}}

\theoremstyle{plainstyle}
\newtheorem{corollary}{Corollary}

\begin{document}

\begin{theorem}
This is a theorem with left/right margins and an indented list:
\begin{itemize}
  \item Indented item
\end{itemize}
More text with adjusted margins.
\end{theorem}

\begin{corollary}
This is a corollary with normal margins:
\begin{itemize}
  \item This list is not indented.
\end{itemize}
\end{corollary}

\end{document}