A simple trick to decorate Theorem-like environments with poker suits QED symbols.
I did not come up with this theorem decoration style (I've first seen it here) nor with the whole code (I salvaged it from TeX StackExchange and other sources over the years). This is just my current implementation of the code.
The XITS fonts provide a Times-like serif typeface for mathematical and scientific publishing. They provide a version of the STIX fonts enriched with the OpenType MATH extension, making them suitable for high quality mathematical typesetting with XeTeX and LuaTeX. XITS fonts are free and open source.