I know you can scroll down to the bottom of the page and change it, but it returns to normal when you come back to the site.
There's an easier way to keep it in Oldschool. Use Safari. For some reason forge hub is in its old skin on safari, so if it's that big of a problem to you just switch internet browsers.
Ew. I just switched back to the old one. Far too bright for me now.
Edit: I suspect it is because the site does not fully support Safari to a certain extent.
It think he meant if you use safari then change it, it stays that way. I like the newer one better anyways, even if I do get a mac (hopefully this christmas)