Shifting \newtheorem

Question

Is there a way to shift a theorem from amsthm ? I mean instead of having :

theorem 1.1 (title): BLABLABLA

You would have theorem 1.1 a little bit on the left and blablabla in the same place. Similarly, it would be something like this : (but without tcolorbox, here I don't need the rest of the functionnalities, the style of amsthm is sufficient)

I want that my definition ( which is for now classic ) has the same behavior as the green and orange box. The dimension is not a problem (I'll set the value of shifting )

Answer 1

You can try something like this. The main point is that the last argument to \newtheoremstyle allows you to put arbitrary code to format the heading. You can make the heading stay within the margins by adjusting \leftskip at the same time you set the body font, i.e. in the third argument to \newtheoremstyle.

\documentclass{article}

\usepackage{amsthm}
\usepackage{lipsum} % for dummy text

\newtheoremstyle{side}{}{}{\advance\leftskip3cm\relax\itshape}{}
{\bfseries}{}{0pt}{%
\makebox[0pt][r]{%
  \smash{\parbox[t]{2.5cm}{\raggedright\thmname{#1}%
  \thmnumber{\space #2}\thmnote{\newline (#3)}}}%
  \hspace{.5cm}}}

\theoremstyle{side}
\newtheorem{theorem}{Theorem}[section]

\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}

\begin{document}

\lipsum[1]

\begin{theorem}
  \lipsum[1]
\end{theorem}

\begin{theorem}[Euclid]
  \lipsum[2]
\end{theorem}

\begin{theorem}[Very long name]
  \lipsum[3]
\end{theorem}

\begin{definition}
  \lipsum[1]
\end{definition}

\end{document}