Yes, a WordPress theme can be as simple as a child theme that includes only a single css file with some css overrides, or a full fledged Theme Framework which includes copious amounts of php and more…or they can be just your normal theme(which generally contains css, possibly javascript, and php files that might utilize php only to integrate with the content WordPress provides into an html document that is embedded in the php file).
The best way to start out playing with themes is to begin making child themes based off a theme like twentyten or twentyeleven made by WordPress.
solved Are WordPress templates really just HTML and CSS? [closed]