OS_X_Aqua can be adopted to your own forums by viewing the source for their CSS. Copy it and place it in to a new style on your forum with the entire bit being placed in to the "Extra Properties" section. If there are images linked within that style, download those, and adjust the links within your new style, to match the location where you've stored the images.
Thats one route. The other is to email the forum admin and ask if he is willing to share the source for his style, or if he will share the archive of it.
Using a bit of google-fu, you probably could also find it on a google search.
