[Tex/LaTex] changing font style/size for proof, adding a square after example

environmentsfontsfontsizesymbols

Is is possible to change font size/(or style) for a proof? I was also thinking to add a small hollow square at the end of every example to separate the text a little. Something similar that works like \qedhere symbol. 

\documentclass[11pt,a5paperfootinclude=true,headinclude=true]{scrbook} % KOMA-Script book
\usepackage[T1]{fontenc}
\usepackage{lipsum}
\usepackage[linedheaders,parts,pdfspacing]{classicthesis} % ,manychapters
\usepackage[bitstream-charter]{mathdesign}
%\usepackage[osf]{libertine}
\titleformat{\section}
 {\normalfont\bfseries}{\thesection}{1em}{}
\usepackage{amsthm}
\usepackage{amsmath}
 \usepackage{multicol}
 \usepackage{IEEEtrantools}
\usepackage{anysize}
\usepackage{stmaryrd}
\usepackage{bbding}
\usepackage{chngcntr}%added to reset footnote for each chapter
\counterwithout{footnote}{chapter}%added to reste footnote for each chapter 
\marginsize{0.1\paperwidth}{0.1\paperheight}{2cm}{2cm}
\newcommand{\bigslant}[2]{{\raisebox{.2em}{$#1$}\left/\raisebox{-.2em}{$#2$}\right.}}
\usepackage{enumerate}
\theoremstyle{plain}
\newtheorem{thm}{Theorem}[chapter] % reset theorem numbering for each chapter
\theoremstyle{definition}
\newtheorem{defn}[thm]{Definition} % definition numbers are dependent on theorem numbers
\newtheorem{exmp}[thm]{Example}
\theoremstyle{corollary}
\newtheorem{cor}[thm]{Corollary}
\theoremstyle{lemma}
\newtheorem{lem}[thm]{Lemma}
\theoremstyle{proposition}
\newtheorem{prop}[thm]{Proposition}
\newcommand{\ndiv}{\hspace{-4pt}\not|\hspace{2pt}}
\DeclareMathOperator{\disc}{disc}
%\renewcommand{\qedsymbol}{\rule{0.7em}{0.7em}}
\renewcommand\qedsymbol{\RectangleBold}
\def\quotient#1#2{%
    \raise1ex\hbox{$#1$}\Big/\lower1ex\hbox{$#2$}%
}
\DeclareMathOperator{\order}{\mathcal{O}_{K}} %defined myself
\DeclareMathOperator{\z}{\mathbb{Z}} %defined myself
\DeclareMathOperator{\q}{\mathbb{Q}} %defined myself
\DeclareMathOperator{\C}{\mathbb{C}} %defined myself
\begin{document}
\begin{proof}
This is my proof
\end{proof}
\begin{exmp}
Want a hollow square at the end of this example.
\end{exmp}
\end{document}

Which produces

enter image description here

Best Answer

It's easy with thmtools:

\documentclass[
  11pt,
%  a5paper,% really?
  footinclude=true,
  headinclude=true
]{scrbook} % KOMA-Script book

\usepackage[T1]{fontenc}
\usepackage[linedheaders,parts,pdfspacing]{classicthesis} % ,manychapters
\usepackage[bitstream-charter]{mathdesign}
\usepackage{bbding}
\usepackage{amsthm}
\usepackage{thmtools}

\theoremstyle{plain}
\newtheorem{thm}{Theorem}[chapter] % reset theorem numbering for each chapter
\newtheorem{cor}[thm]{Corollary}
\newtheorem{lem}[thm]{Lemma}
\newtheorem{prop}[thm]{Proposition}

\theoremstyle{definition}
\newtheorem{defn}[thm]{Definition} % definition numbers are dependent on theorem numbers

\declaretheoremstyle[
  style=definition,
  qed=\openbox,
]{example}
\declaretheorem[
  name=Example,
  style=example,
  numberlike=thm,
]{exmp}

\renewcommand\qedsymbol{\RectangleBold}

\begin{document}
\begin{proof}
This is my proof
\end{proof}
\begin{exmp}
Want a hollow square at the end of this example.
\end{exmp}
\end{document}

I have removed the inessential parts of the preamble. Note that there is no lemma, corollary or proposition theorem style. A full specification with thmtools would be

\documentclass[
  11pt,
%  a5paper,% really?
  footinclude=true,
  headinclude=true
]{scrbook} % KOMA-Script book

\usepackage[T1]{fontenc}
\usepackage[linedheaders,parts,pdfspacing]{classicthesis} % ,manychapters
\usepackage[bitstream-charter]{mathdesign}
\usepackage{bbding}
\usepackage{amsthm}
\usepackage{thmtools}

\declaretheorem[
  style=plain,
  name=Theorem,
  within=chapter,
]{thm}
\declaretheorem[
  style=plain,
  name=Lemma,
  numberlike=thm,
]{lem}
\declaretheorem[
  style=plain,
  name=Proposition,
  numberlike=thm,
]{prop}
\declaretheorem[
  style=plain,
  name=Corollary,
  numberlike=thm,
]{cor}

\theoremstyle{definition}
\declaretheorem[
  style=definition,
  name=Definition,
  numberlike=thm,
]{defn}

\declaretheoremstyle[
  style=definition,
  qed=\openbox,
]{example}
\declaretheorem[
  name=Example,
  style=example,
  numberlike=thm,
]{exmp}

\renewcommand\qedsymbol{\RectangleBold}

\begin{document}
\chapter{Title}
\begin{prop}
A proposition.
\end{prop}
\begin{lem}
A lemma.
\end{lem}
\begin{thm}
A theorem.
\end{thm}
\begin{proof}
This is my proof
\end{proof}
\begin{cor}
A corollary.
\end{cor}
\begin{exmp}
Want a hollow square at the end of this example.
\end{exmp}
\end{document}

enter image description here

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