I want to be able to say that the registration page or login page use the skin of the forum the user was currently in, and furthermore be re-directed BACK to that forum as opposed to the root of the forums.

I think this would be an excellent feature

you change your skin when you register?

Strange Will is trying to suggest that the registration page remembers what forum the user was in just before they click on the registration link.

And accordingly:
1) displays the registration page using the skin that is assigned to that particular forum (if there is not assigned anything in particular, it would use the default skin);
2) redirects the user back to that particular forum after the registration data is submitted.

I think it's not something very necessary, but it would be a nice user-friendly addition.

This suggestion doesn't really make sense. When you register, you are calling up a "global" page - that is, the reg page. When you've completed your registration, you expect to be taken to the login page, or be logged in, then taken to the board index as a starting point. It would just feel plain unnatural to be taken to some forum that you probably don't even remember you were in in the first place.

Well my users seem to very easily get lost when they get kicked out of the forum they thought they were logging into, because I do run a couple of small sub groups on my forum.

Again, this would be an option, probably just a variable sent through a get command that said what forum id they came from, if the option is on, load the skin and when you're done go back to that forum (also with forum logins, should go back to the forum)

Anyway, just an option, nothing needed greatly, but I think it would be a nice feature.

Yes, it does make sense for boards that are accessible also for non-registered users.

An example. A user finds an interesting thread via a search engine and then wants to post something in it but it turns out they need to register to post. At this point (while the thread is open) the user clicks on the registration link and fills and submits the form. Now the user is redirected to the index page and it may not be very easy to find that particular thread again especially if the board is huge.

If it was a thread, perhaps it would make more sense, yes.

But this would require quite big changes to be made in the code. First the register link would have to capture the referer (which isn't always sent anyway), then that value would have to be a hidden input at every stage of the registration process (coppa, accepting the terms etc.), then it would probably be best to give them a couple of links at the end of the registration process asking them where they want to go.

Thing is though, there's always the back button

Just pass on the &forumid=1 variable, it will be resubmitted (form action) and then decision will be made by the software.

It's a sloppy way though... your way is cleaner