Lumen is an experimental theme which provides limitless possibilities. To take full advantage, a good working knowledge of CSS/(x)HTML is required. The presentation can be changed completely based on many factors.
Partial list of features
Each template file (.tpl.php) overflowing with relevant classes. This gives the possibility to change the look of each element (or whole page) from any bit of information contained therein. It's also smart about moving node classes up to the body when showing a full node.
Classes, loading of style sheets and template naming suggestions based on taxonomy and internal path information. -these three features work closely together. The latter two being optional.
Override module styles (core & contrib) by placing copies into the "styles_module" folder or disable them completely by appending ".disabled" to the file name. -makes it easier to apply properties by not having to use cascade rules to override styles that are already set.
jQuery plug-in included for UI customizations. -plays a big role
There's a lot more to this theme. The code is commented heavily. Too see all the theme overrides and custom functions, look inside the API pages.
It's still under heavy development and unstable. By the time Lumen reaches a 1.0 release, it may contain many changes. This is just a warning! Do not use in a production environment.