Add mouseover event to main-navigation in Tremont template

I'm working on a site using the Tremont template see here

I would like to transfer the mouseover effects of the index-navigation and apply these to the main-nav at the top right of the screen so as when you hover over the menu items they change the background video/image see here.

I have achieved a similar effect on other projects but am having difficulty with 1. targeting the background video and 2. placing the images I wish to replace it with.

Thanks in advance, any help would be greatly appreciated as I've been battling with this one for too long.