Information now available on the website.
http://www.seagulls.co.uk/news/article/2016-17/young-seagulls-brighton-hove-albion-3446709.aspx
It seems that parking is £5 if pre-booked or £10 on the day.
There's free parking in Car Park A from 1pm onwards for the Santa Dash. Don't think this helps for the Young Seagulls party though, as it's earlier in the day.