JQuery SelectBox - adding an .active class when expanded

JQuery SelectBox - adding an .active class when expanded

Hello, I've been trying to understand how to select and style jUqey's SelectBox when it is expanded. There seems to only be an ARIA attribute,  aria-expanded='false' / 'true' but targeting the SelectBox this way doesn't seem correct and I haven't been able to apply styles this way. 

Does anyone have any insight into targeting the SelectBox while it is expanded for CSS styling?