Tweaking \theoremstyle

Question

Well I want to do is when LaTeX writes Theorem at the new line instead of the left side of the page it be a little indented. In other words I want it to start from a little bit to the right.

Answer 1

Use the ntheorem package.

\usepackage{ntheorem}
\setlength\theoremindent{\parindent}
\newtheorem{thm}{Theorem}

Now the thm environment will be indented the same amount as a paragraph.

Answer 2

Here's a solution using etoolbox and the addmargin environment of the scrextend package (which is included in KOMA-script):

\documentclass{article}

\newtheorem{theorem}{Theorem}

\usepackage{etoolbox}
\usepackage{scrextend}
\pretocmd{\theorem}{\bigskip\begin{addmargin}[1.5em]{0pt}}{}{}
\apptocmd{\endtheorem}{\end{addmargin}\bigskip}{}{}

\usepackage[english]{babel}
\usepackage{blindtext}

\begin{document}

\blindtext

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

\blindtext

\end{document}

Answer 3

one possible solution here, maybe not the best. If you are happy with it, you may want to control the indentation with the line

\def\theoremframecommand{{\hspace{10pt}}}

minimal exmaple is:

\documentclass[fleqn]{book}
\usepackage{lmodern}
\usepackage{xcolor}
\usepackage[top=3cm,left=3cm,right=3cm,bottom=2.5cm,a4paper]{geometry}
\usepackage{amsmath}
\usepackage{framed}
\usepackage[amsmath,thmmarks,framed]{ntheorem}
\usepackage{lipsum}

\theoremstyle{plain}
\theorembodyfont{\normalfont}
\setlength\theorempreskipamount{0pt}
\setlength\theorempostskipamount{-3pt}
\def\theoremframecommand{{\hspace{10pt}}}
\newshadedtheorem{Theorem}{Theorem}[chapter]

\begin{document}
\section{chapter 1}
\section{section 1}
\lipsum[5]
\begin{Theorem}[Theorem 1]
\lipsum[5]
\end{Theorem}
\lipsum[5]
\end{document}