How to use thmtool to defined a bolded theorem style?

Question

I am currently following this answer to create a theorem style using theorem tool. How to change the way theorems are displayed?

Previously I used something from the amsthm package, but it had a major conflict with my template, therefore all my precious theorem styles were destroyed

The style I am looking for looks like this:

Does anyone know how to change the line "\declaretheoremstyle[headfont=\scshape,postheadspace=\newline]{mystyle}" to generate the theorem style I want?

Answer 1

Like this?

\documentclass{book}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage{amsthm, thmtools}
\usepackage{cleveref}
%
\declaretheoremstyle[%
headfont=\normalfont\bfseries,
within=chapter,headpunct={.\smallskip \newline}, 
bodyfont =\itshape, 
spaceabove = 8pt,spacebelow = 8pt]%
{mythmstyle}

\declaretheorem[style=mythmstyle]{Theorem}

\begin{document}

\setcounter{chapter}{3}
Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text.
\begin{Theorem}\label{testthm}
  The following assertion is true: %
  \begin{equation}
    a = a\end{equation}
    \end{Theorem}

    We see in \cref{testthm}

\end{document}