How to customise (e.g. make thicker) the end-proof symbol in Beamer?

Question

I would like to change the symbol for the end of proofs (the square) used in Beamer. It is quite thin, not very visible in some colorthemes. How can I have a thicker border and still in the structure color?

Answer 1

Beamer defines the end-proof symbol in this way:

\def\qedsymbol{\leavevmode\hbox{\usebeamertemplate*{qed symbol}}}

therefore it is possible to change it by means of something like:

\setbeamertemplate{qed symbol}{...}

Defining a new symbol with TikZ:

\newcommand{\bigqed}{\tikz[baseline]\draw[ultra thick,structure.fg](0,0)rectangle(0.275,0.275);}

it becomes:

\setbeamertemplate{qed symbol}{\bigqed}

Complete example:

\documentclass{beamer}
\usetheme{Boadilla}
\usepackage{tikz}

\newcommand{\bigqed}{\tikz[baseline]\draw[ultra thick,structure.fg](0,0)rectangle(0.275,0.275);}

\begin{document}

\begin{frame}
\setbeamertemplate{qed symbol}{\bigqed}
\begin{proof}
\begin{itemize}
\item First item.
\item Second item.
\item Third item.
\end{itemize}
\end{proof}

\end{frame}

\end{document}

Result:

Notice that in order to maintain the structure color of the current theme, the command uses structure.fg.