Accordion Menu - Where is the original CSS file of the module?
The Accordion Menu module uses jQuery UI, which is included with core. If the styles you're looking for are in a CSS file, they'll be in /misc/ui/jquery.ui.accordion.css (or possibly one of the more general jQuery UI CSS files).

Bear in mind that jQuery UI is a javascript library, so the styles might be applied directly to the elements at runtime, and may never exist in a CSS file as such.