I would call customizing a WordPress Theme simply doing the bare minimum.
I would be obtaining all of the information from the ‘client’ such as:
- Site Name
- Site Logo
- Desired Slide Show Images
- etc.
Basically just changing the theme up to fit the client’s needs.
If they need you to edit the HTML and PHP code, it will require more time, energy, and knowledge than that of a designer. You should be distinguishing these two types of services. Editing the source files means you might create bugs that you will not be able to go to the theme provider to get help with. A good programmer can do it, sure. But, don’t let it be part of a simple theme customization.
Just my thoughts.