How to replace browser based popup alerts with modal / light box on bidding page

Hi guys,

I am using AuctionPress Responsive theme.
Wondering if anybody has got a solution to avoid the browser based popup alerts generated by the script when someone try to bid with out signing in (see first screenshot). Is there a way to redirect to login page instead?

Also for any other browser based popup alerts (bidding on own listing etc), how do I change to light box or modal popups alerts instead? Something similar to what you get to confirm bid (see second screenshot)

Many thanks for your help in advance !

Kind Regards