My aim is to write the body text and only the body with a big left skip; theorems, titles and display math starting from the very left, without skips.
I use the command \leftskip=4cm, which works good on the body ignoring all the environments, but works also for titles. I'm putting it into the preamble.
What do you suggest me?
Although LaTeX is the best in organizing text, I want it to write my personal notes and I absolutely need to use my personal typeset.
EXAMPLES
Here is an example of what I've done till now (I hope not-English examples are allowed, just for the sake of typesetting):

and here is an example of what I'd like to make:

Here is a minimal executible code:
\documentclass[a4paper]{book}
\usepackage{geometry} %typography
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
% THEOREMS
\newtheorem{teo}{Teorema} %theorems: unindented environment
\newtheorem*{dimostratio}{Proof}
\newenvironment{proof.} %proofs: indented environment
{\begin{dimostratio} \leftskip=4.5cm \mbox{} \noindent\ignorespaces }
{\end{dimostratio}}
% TYPOGRAPHY
\geometry{a4paper, top=2cm, bottom=2cm, left=2cm, right=2cm, bindingoffset=5mm}
\leftskip = 4.5cm % indentation of everythingh isn't a theorem.
%DOCUMENT
\begin{document}
Here is the indented body. %indented as I like.
\lipsum[1]
\section{Indented (sic) title} %indented but I don't like.
\lipsum[2]
\begin{teo} Here is an unindented environment, as I like.
\end{teo}
\begin{proof.}
And here is an indented environment, as I like.
\end{proof.}
\lipsum[4]
\end{document}