The following customization tutorials are not theme specific, but give you real world examples on theme customization.
Adding Olark Live Chat to your Website
Adding text to the checkout when a specific item is being purchased
Changing i18n checkout text within the template
Creating an extended description longer than 2000 characters
Fix insecure content warning due to template modification
Hide Update and Continue Shopping Button on Single Page Checkout
Implementing Amazon.com Conversion Pixel
Porting IfPurchased Tokens on the legacy receipt over to Velocity
Apply a specific Font Family to throughout a theme via CSS