To override the existing text spree mailers, copy them into your application:
cp -r /path/to/spree/core/app/views/spree/{order,shipment}_mailer app/views/spree/
To add HTML versions create the following
- app/views/spree/order_mailer/confirm_email.html.erb
- app/views/spree/order_mailer/cancel_email.html.erb
- app/views/spree/shipment_mailer/shipped_email.html.erb