The CSS and images that control the Studio look and feel are located in the html/css/eid-default directory.
For WebLogic Server deployments, the directory is embedded in endeca-portal-<versionNumber>.war, which in turn is embedded in endeca-portal-weblogic-<versionNumber>.ear.
This files are all minified.
These are the unminified versions of the endeca-skin-idNumber.css files.
Note that while this directory contains both minified and unminified versions of the same CSS files, by default, Studio uses the minified version. If you update the unminified version, then you also must configure Studio to use that version.