If by minimal you mean the fewest distractions on the home screen, the the Zen theme with a plain background would be best. If you want a basic theme with just a few necessary apps on the home screen, then probably the Vodafone Today theme.
Zen with minimal background:
http://i39.photobucket.com/albums/e1...8/BBOrange.jpg
Vodafone Today:
http://www.blackberryforums.com/atta...2&d=1149701580
Minimal Theme?
