Attaching 'active' class to faqfield expanded question
This functionality is provided by FAQ Field using the jQuery Accordion UI. 

In Structure > Content Types > Your Content type > manage display (admin/structure/types/manage/[your_content_type]/display), if you change your FAQ field display settings from Definition List to Accordion, an up/down caret is shown next to each question. 



If you click on the gear icon corresponding to your FAQ Field you can configure which question is active by default, as well as whether the questions are collapsible. 



Result:



The Accordion setting also adds the class ui-state-active to the active question element (an h3) and the class ui-accordion-content-active to the active answer element (a div).