In reply to William Pitcher.

@William That person could be an implementer (using Tom's description), but they're likely to learn some Javascript and PHP or at least aspects of the WP framework and/or a theme framework.

While this may seem like a very limited skillset (it is), it can have some unique benefits since it's restricted to purely presentational customization. A lack of custom code and dependencies that aren't updated with the core or a third party plugins makes a very stable, durable, and secure site if the theme/framework and plugins are good.