Most of Drupal theming is XHTML and CSS. They'll also need some basic PHP knowledge....the more PHP they undersatnd the more they'll be able to do.
Beyond those basic starter skills, they'll obviously have to learn how drupal works. I'd have them check out the Drupal Theme Guide to get started on that stuff.
If I were to ask church members to help with theming our Drupal-based site, what skills should I ask for?
(That is, I don't think most of them have heard of Drupal, so asking for help in "Drupal theming" would probably not work.)
BTW, I will definitely refer them to the resources covered in this recent post:
http://geeksandgod.com/forums/dynamic-website-foru...
Many thanks for any help!,
Justin