Theming is different from Designing. It is another layer of Web Development, working mostly with CSS, JS, PHP and Drupal coding standards, to translate existing PSD design or mockup design into a functional HTML, hence a Drupal theme. My client, personal or third party provider, gives me the PSD design, sometimes mere instructions or rough mockups, and I translate them into a functional Drupal theme. Communication and detailed job descriptions are the key to succesful theming.