Most WordPress developers unable to understand the basic difference between the theme customization & theme design creation. But why so happens? This is because of the fact that they are concentrated fully upon running their business not upon the dictionary of WordPress design.
There are number of professionals using WordPress and the number is still increasing, which therefore, is making this topic even messier. From the following content the clarity can be derived.
Theme customization vs. Custom WordPress Design Theme
The basic difference between theme customization & custom WordPress theme is the graphic designer basically. A true designer is professionally trained in the art of design. More