Hanging indent in an amsthm theoremstyle

Question

I'm aiming to have a theoremstyle which has hanging indentation, where all lines after the first line are indented by some amount of space. I've searched for previous answers to similar questions, but most of these do not use the amsthm package, or are difficult for me to understand. My current theoremstyle is as follows:

\newtheoremstyle{mystyle}%
{3pt}
{3pt}
{}
{}
{\bfseries}
{ --}% Punctuation after theorem head
{.5em}% Space after theorem head
{}% Theorem head spec (can be left empty, meaning ‘normal’)

This looks like

which does not contain any sort of indentation currently.

I would like something which looks like this

which I have poorly approximated with an additional quotation environment inside. However, in doing so, the spacing is not correct. Hopefully, the idea is clear--the text should actually be formatted with a proper hanging indent.

Is there some sort of proper "hanging indent" setting I can put into the indentation parameter of the theorem style? Or do I have to do some more manual work? I would prefer not to replace amsthm (I'm trying to rewrite documents already made with amsthm), but having an additional package is not problematic.

Answer 1

Your question is very similar to this one: Theorem environment with hanging indentation.

Only the theorem layout changes a bit.

Here is the analogous of this solution with your theorem style.

\documentclass{article}
\usepackage{amsmath}
\usepackage{ntheorem}
\theoremstyle{plain}
\theoremseparator{ --}
\theorembodyfont{\normalfont}
\theorempreskip{3pt} 
\theorempostskip{3pt} 
\theoremindent15pt
\theoremheaderfont{\normalfont\bfseries\hspace{-\theoremindent}}
\newtheorem{theorem}{Theorem}

\usepackage{mwe}% for testing purpose only

\begin{document}
\blindtext
\begin{theorem}
\blindtext
\end{theorem}
\blindtext
\end{document}