Copy the default theme from
[sfPropelPlugin]data/generator/sfPropelCrud/default/theme
to
data/generator/sfPropelCrud/default/theme
Edit the files in data/generator/sfPropelCrud/default/theme/templates as you would with an admin template.
Generate like usual: ./symfony propel:generate-crud [app] [module] [base]
Derived from: http://www.playingwithwire.com/2007/02/review-of-the-definitive-guide-to-symfony/