Customization in consumer products has always been targeted at "making it yours". We're all unique so even if we all have an iPhone and we're all on Facebook, we like that unicity to shine through somewhere. Some services used to overdo that (ahem myspace ahem). Facebook and Twitter on the other hand have a nice balance of offering customization, allowing you to show your personality on your profile page, while keeping the usability and look & feel of the service in tact.
This is true as well for websites. If you're going to create a website for a business, they'll definitely feel they're unique and they are going to want it to look like it's theirs. And they're going to want to feel like they had a say in how it looks.
So picking a theme is never going to be sufficient. Even if there are a hundred themes, any theme you choose a customer will want to customize at some level.
What options you should offer depends on many factors. For instance, you could offer only one or two themes that can be further customized with images and colors. Just explain the limitations get you the best set of features for the lowest price and that further customization would be more expensive. And at the other end of the spectrum you could do without themes at all and build everything from scratch.