Question

As the title says I need to define a QED symbol automatically (preferable a colorbox) at the end of my custom newtheorem.

Any suggestions would be appreciated.

Answer 1

I’m not sure what you mean. If you use the package amsthm you have the command \qed to put a QED wherever you want.

You can customize the \qed by changing the command \qedsymbol. So you would do something like

\usepackage{amsthm}
\renewcommand{\qedsymbol}{$\heartsuit$}
\newtheorem{thm}{Theorem}
\newenvironment{\mythm}{\begin{\thm}}{\qed \end{thm}}
...
\begin{mythm}
blah
\end{mythm}

This if you want the QED symbol to be the same that appears in proofs. If not you can do simply

\newtheorem{thm}{Theorem}
\newenvironment{\mythm}{\begin{\thm}}{\hfill $\heartsuit$ \end{thm}}
...
\begin{mythm}
blah
\end{mythm}

Of course you can change \heartsuit with whatever you want.