with javascript, there is no way to import graphic styles from libraries or files. there is a way to open ai documents as graphic style libraries but then there is no script access to that library.

currently what i have to do is have an ai file with objects with graphic styles assign to them. with my script i open that file, select the objects, copy them, close the file, switch documents, paste objects, delete objects.

this can take a while and it is a very ugly solution. ideally there should be something as simple as importGraphicStylesFromFile(File)

or at least
GraphicStyleLibrary("name of library").graphicStyles.getByName()