hafrogman
Adventurer
Themes were originally introduced in the Dark Sun Campaign Setting book, then later expanded into less setting specific additions through Dragon articles, some of the 'Essentials' books (Heroes of the Feywild, Heroes of Elemental Chaos) and some of the other more recent publications.I'm not immediately familiar with "themes." Where can I read about them?
They are a little like a minor-multiclass. Each grants a power (usually an encounter power) and the ability to (at later levels) substitute the theme's power for your normal class power selection.
There was a preview article of the DSCS that covers the hows and whats of them here.