Numbering Theorems/Definitions in Lyx according to Subsection, Line Breaks after Text

Question

I am currently using Document > Settings > Modules > Theorems (Numbered by Type within Sections) in Lyx.

However, I need my theorems/definitions to be numbered according to subsection instead. i.e.

2 Introduction

2.1 Subsection ...

Definition 2.1.1

Theorem 2.1.2

2.2 Subsection 2

Theorem 2.2.1

Additionally, I also want the text after the labels to be in a new line (instead of continuing after the bold text), i.e.

Definition 2.2.2

The definition of formula is...

I would also like to state that I am rather new to lyx so I am not familiar with editing TeX code (if required) in the editor so detailed steps would really be appreciated.

Thank you!

Edit 1: MWE (I removed mostly font related code)

% Preview source code

#LyX 2.2 created this file. For more info see http://www.lyx.org/
\lyxformat 508
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass article
\use_default_options true
\begin_modules
theorems-ams-bytype
theorems-sec-bytype
\end_modules
\maintain_unincluded_children false
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\paperfontsize default
\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_package amsmath 1
\use_package amssymb 1
\use_package cancel 1
\use_package esint 1
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 1
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 1
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header

\begin_body

\begin_layout Standard
\noindent
\begin_inset CommandInset toc
LatexCommand tableofcontents

\end_inset


\end_layout

\begin_layout Standard
\paragraph_spacing onehalf
\noindent
\begin_inset Newpage newpage
\end_inset


\end_layout

\begin_layout Section
\paragraph_spacing onehalf
\noindent
Title
\series bold
 1
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
numberwithin{equation}{section}
\end_layout

\end_inset


\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
setlength{
\backslash
parindent}{1.5pt}
\end_layout

\end_inset


\end_layout

\begin_layout Section
\paragraph_spacing onehalf
\noindent
Title 2
\end_layout

\begin_layout Subsection
\paragraph_spacing onehalf
\noindent
Sub 1
\end_layout

\begin_layout Subsection
\paragraph_spacing onehalf
\noindent
Sub 2
\end_layout

\begin_layout Definition
\noindent
My first definition
\end_layout

\end_body
\end_document

Answer 1

I do not know whether it's possible from within LyX, but here is what you have to add to you preamble to obtain it:

\newtheoremstyle{break}%
{5pt}{4pt}% spaces added before and after the environment
{\itshape}{}%
{\bfseries}{}% % Note that final punctuation is omitted.
{\newline}{}
\theoremstyle{break}
\newtheorem{theorem}{Theorem}[subsection]

\newtheoremstyle{defbreak}%
{5pt}{4pt}% spaces added before and after the environment
{\upshape}{}%
{\bfseries}{}% % Note that final punctuation is omitted.
{\newline}{}
\theoremstyle{defbreak}
\newtheorem{defn}{Definition}[subsection]

Here is the full code (slightly modified):

\documentclass{article}
\usepackage{amsthm}
\usepackage{lipsum}

\newtheoremstyle{break}%
{5pt}{4pt}% spaces added before and after the environment
{\itshape}{}%
{\bfseries}{}% % Note that final punctuation is omitted.
{\newline}{}
\theoremstyle{break}
\newtheorem{thm}{Theorem}[subsection]

\newtheoremstyle{defbreak}%
{5pt}{4pt}% spaces added before and after the environment
{\upshape}{}%
{\bfseries}{}% % Note that final punctuation is omitted.
{\newline}{}
\theoremstyle{defbreak}
\newtheorem{Def}{Definition}[subsection]

\begin{document}

\section{Introduction}
\subsection{A first subsection}
\lipsum[3]

\begin{thm}
  A silly test theorem. A silly test theorem. A silly test theorem. A silly test theorem. A silly test theorem.
\end{thm}

\lipsum[4]
\begin{Def}
 This is a \texttt{defbreak} style example. Some text. Some more text. Some more text. Some more text.
\end{Def}
\lipsum[5]

\end{document}

Answer 2

A good solution would require make your own module, but a poor's man solution could be:

1) Not load any module.

2) Insert this in Document > Stetings... > LaTeX Preamble

\usepackage{amsthm}
\newtheoremstyle{xxx}{}{}{\itshape}{}{\scshape\bfseries}{:}%
{\newline}{\thmname{#1}\thmnumber{ #2}\thmnote{(#3)}\smallskip}%
\theoremstyle{xxx}
\newtheorem{xxx}{\protect\theoremname}[subsection]
\providecommand{\theoremname}{Smart theorem}

3) Make your your theorem with \begin{xxx} ... \end{xxx} in a ERT box (Ctrl+L) or alternatively make two ERT boxes for \begin{xxx} and \end{xxx} respectively and left the theorem text as a normal paragraph.

The result must be equivalent to this LaTeX source code:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{amsthm}
\newtheoremstyle{xxx}{}{}{\itshape}{}{\scshape\bfseries}{:}%
{\newline}{\thmname{#1}\thmnumber{ #2}\thmnote{(#3)}\smallskip}
\theoremstyle{xxx}
\newtheorem{xxx}{\protect\theoremname}[subsection]
\providecommand{\theoremname}{Smart theorem}
\begin{document}
\section{xxx}
Bla bla bla ...
\subsection{adasd}
\begin{xxx}
Bla bla bla ...
\end{xxx}
Bla bla bla ...
\end{document}

Obviously, you can change the style xxx to some name more meaningful, as well as the arguments of \newtheoremstyle to fit more to your desired layout. Run texdoc amsmath for information about the arguments.