The admin area can definitely be themed, using either CSS, a plugin, or a combination of both. To get started, read the Codex page on Creating Admin Themes.
To expand further – any overriding of core WP functions must be done from a plugin. It’s fairly simple, and just like any other function override – either adding the content with a new function and add action or with a filter to replace the existing function with your new function using add_filter