I am a great fan of Freecell solitaire card game that is part of aisleriot package. There are some built-in card themes, but one can install a custom set as well. Steps to do so:

  1. Download the theme (uncompressif necessary)
  2. Copy the svgz file to /usr/share/aisleriot/cards/ folder.

Here are my favorites: dark version of C1 card set (C1B) from gnome-look.org and Sin city theme:

Sin city screenshot of card themes