Do all your oldd urls follow the same date pattern? and do all your theme folders stay named the same? If so this would work with the redirect from old to new.
# redirect /2005/12/30/widget-ideas/ ---> /themes/themes2/widget-ideas/
RewriteRule ^[0-9]+/[0-9]+/[0-9]+/([^/]+)/?$ http://www.yourdomain.com/themes/themes2/$1/ [R=301,L]
You'll probably want this to be the first rewrite rule in the htaccess.