Having done something similar a few years back (CFForum ---> vBulletin), I'm not gonna yell too loudly about how long it took, or much of anything like that.
As for green, blue, whatever ... I've always thought it would be a Good Thing(tm) for sites to allow you a choice of stylesheets ... but I've not yet been able to solve that on our properties ... so I'm not sure how much I should expect it on anyone else's ....