To develop for liferay, we create separate plugin projects. These are used to create portlets or hooks (overrides on Liferay core), layout templates, and another type is the theme.
So a coder will create a new theme plugin project, define a parent theme to inherit from, then add override files to replace the parent theme class definitions with their own.
No, there are not a lot of themes like there are with wordpress. Wordpress is really targeted towards individuals, not corporations, so they have lots of themes floating around out there for you to take advantage of.
Liferay, on the other hand, is really more for the enterprise (IMHO). There, the enterprise doesn't just pick some off the shelf theme, they typically already have a color palette, images, and often an existing website they want the Liferay theme to mimic.
Liferay offers as part of the core two foundation themes (unstyled and styled, you use the first when you want to write everything for the theme or the second if you want a basic starting point) and the classic theme. The classic theme is very simple, and it is meant to show off a few Liferay features but not be the center point of a Liferay demo.
From a marketplace perspective, there's just no money in creating wordpress-like themes, as they really won't do well. Enterprises might like some of the things in the MP theme, but in the end they're going to want it customized for their needs. So at most the MP themes become examples (i.e. the Zoe themes) or "marketing" (showing what the company/consultant can do and they would bring their skills to your theme project).
