If you are changing CSS through javascript, you can use Mutation Events (Second Level Events).
You can listen on those events :
- DOMAttrModified
- DOMAttributeNameChanged
- DOMCharacterDataModified
- DOMElementNameChanged
- DOMNodeInserted
- DOMNodeInsertedIntoDocument
- DOMNodeRemoved
- DOMNodeRemovedFromDocument
- DOMSubtreeModified
So either listen on DOMAttrModified, with Style attribute, or Class attribute.