Functionality of expand and collapse for an accordion(JS) would work as is with regular proxy pattern.(Can check in Preview or disabled/View as Published mode)
CSS needs to be done per the desired look and feel of the project.
In order to cross check if you have created proxy component correctly, you can create a sample project in your local using AEM project maven archetype which has proxy accordion component, you can make use of the same. (Compare what needs to be overwritten when creating proxy out of Core component)