\documentclass[parskip=half]{scrartcl}
\usepackage{amsthm,thmtools}
\usepackage{lipsum}
\usepackage{showframe}
\newlength\mylen
\settowidth\mylen{\normalfont\hspace*{2.5em}}
\declaretheoremstyle[
headfont=\bfseries,
bodyfont=\normalfont\leftskip\mylen,
headindent=-\mylen,
preheadhook=\normalfont,
]{INDENTthm}
\declaretheorem[
within=section,
style=INDENTthm,
name=Theorem
]{thm}
\begin{document}
\begin{thm}
\lipsum[1-2]
\end{thm}
\end{document}