Numbering theorems in the left margin

Question

My question is similar to this and this. I would like to have the theorem number in the left margin (0.6em spacing to the main text) and the Name 'Theorem' should be justified to the main text. The solution should made with the amsthmpackage, because I've made my very specific proof environment with \renewenvironment from amsthm.

\documentclass[12pt,a4paper]{article}
\usepackage[ngerman]{babel}
\usepackage{lipsum,lmodern,libertine}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{amsthm}
\newtheoremstyle{mystyle}
{\topsep}               %space above
{\topsep}               %space below
{}                      %bodyfont
{}                      %indent
{\bfseries}             %headfont
{}                      %punctuation
{0.6em}                 %space after head
{#2 #1 #3}              %theoremheadspec
\theoremstyle{mystyle}
\newtheorem{theorem}{Theorem}

\begin{document}
\lipsum[1] 

\begin{theorem}
Let $f\colon [a,b]\to\mathbb{R}$ be continious and let $F$ be an antiderivative of $f$, then
\begin{align*}
\int_a^b f(x)\,\mathrm{d}x=F(b)-F(a).
\end{align*}
\end{theorem}

\end{document}

This looks like

But it should look like this (this was made with the ntheorem package):

Answer 1

It sounds like you want the \llap command for left overlap

{\llap{#2 }#1 #3}              %theoremheadspec

Here's a complete MWE to play with.

% arara: pdflatex
\documentclass[12pt,a4paper]{article}
\usepackage{amsmath,amssymb}
\usepackage{lipsum}
\usepackage{amsthm}
\newtheoremstyle{mystyle}
{\topsep}               %space above
{\topsep}               %space below
{}                      %bodyfont
{}                      %indent
{\bfseries}             %headfont
{}                      %punctuation
{0.6em}                 %space after head
{\llap{#2 }#1 #3}              %theoremheadspec
\theoremstyle{mystyle}
\newtheorem{theorem}{Theorem}

\begin{document}
\lipsum[1] 

\begin{theorem}
Let $f\colon [a,b]\to\mathbb{R}$ be continious and let $F$ be an antiderivative of $f$, then
\begin{align*}
\int_a^b f(x)\,\mathrm{d}x=F(b)-F(a).
\end{align*}
\end{theorem}

\end{document}