I want to change the look and feel of the post edit page without making changes to any of the wordpress core file, which is basically a theme. For example, for any user with role of 'author', i want to display everything in one column and re-arrange the meta boxes. So is it possible to create a theme for one of the admin pages and how could i get started?
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