cosa pensate ora per quel che riguarda l’impaginazione?
\documentclass[a4paper,12pt,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{mathptmx}
\usepackage[italian]{babel}
\usepackage{latexsym}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{pstricks}
\usepackage{graphicx}
\usepackage{verbatim}
\usepackage{epsfig}
\usepackage{microtype}
\usepackage{indentfirst}
\usepackage{wrapfig}
\usepackage{xcolor}
\usepackage{slashed}
\usepackage{frcursive}
\usepackage{cancel}
\usepackage{color}
\usepackage{fancybox}
\usepackage{numprint}
\usepackage{moresize}
\usepackage[a4paper,top=4cm,bottom=4cm,left=4.5cm,right=4.5cm,bindingoffset=5mm]{geometry}
\usepackage{caption}
\captionsetup{labelformat=empty,textfont=sl}
\usepackage{natbib}
\usepackage[resetlabels]{multibib}
\newcites{web}{Siti Web consultati}
\usepackage{url}
\usepackage{theorem}
\theoremstyle{plain}
\newtheorem{teor}{Teorema}[section]
\theoremstyle{change}
\newtheorem{teo}[teor]{Teorema}
\newtheorem{cor}[teor]{Corollario}
\newtheorem{prop}[teor]{Proposizione}
\newtheorem{lemma}[teor]{Lemma}
\newtheorem{defin}[teor]{Definizione}
\newtheorem{es}[teor]{Esercizio}
\newtheorem{ese}[teor]{Esempio}
\newtheorem{cost}[teor]{Costruzione}
\newtheorem{oss}[teor]{Osservazione}
\newenvironment{proof}{\trivlist\item[]\emph{Dimostrazione \ }\ignorespaces}%
{\nobreak\hskip 1em plus 1fil\nobreak$\Box$
\parfillskip=0pt%
\endtrivlist}
\newenvironment{svolg}{\trivlist\item[]\emph{Svolgimento \ }\ignorespaces}%
{\endtrivlist}
\newcommand{\fncyblank}{\fancyhf{}}
\newenvironment{abstract}%
{\fncyblank \null \vfill \begin{center}%
\bfseries \end{center}}%
{\vfill \null }
\setlength{\unitlength}{2em}
\newcommand{\R}{\ensuremath{\mathbb{R}}}
\newcommand{\Q}{\ensuremath{\mathbb{Q}}}
\newcommand{\Z}{\ensuremath{\mathbb{Z}}}
\newcommand{\F}{\ensuremath{\mathbb{F}}}
\newcommand{\C}{\ensuremath{\mathbb{C}}}
\newcommand{\N}{\ensuremath{\mathbb{N}}}
\newcommand{\PP}{\ensuremath{\mathbb{P}}}
\renewcommand{\leq}{\leqslant}
\renewcommand{\geq}{\geqslant}
\definecolor{lightblue}{rgb}{0.1,124,4}
\newcommand{\asterismo}{\smash{%
\raisebox{-.4ex}{%
\setlength{\tabcolsep}{-.4pt}%
\begin{tabular}{@{}cc@{}}%
\multicolumn2c*\\[-1.5ex]*&*%
\end{tabular}}}}
\newenvironment{changemargin}[2]{%
\begin{list}{}{%
% \setlength{\topsep}{0.1pt}%
\setlength{\leftmargin}{#1}%
\setlength{\rightmargin}{#2}%
% \setlength{\listparindent}{\parindent}%
% \setlength{\itemindent}{\parindent}%
% \setlength{\parsep}{\parskip}%
}%
\item[]}{
\end{list}}
\newenvironment{sistema}%
{\left\lbrace \begin{array}{@{}l@{}}}%
{\end{array}\right.}
\usepackage{fancyhdr}
\pagestyle{fancy}
\begin{document}
\end{document}
fermo restando che devo ancora modificare la dichiarazione dei teoremi con il pacchetto amsthm, come da vostro consiglio!