I have forced everyone back to the Core theme. you can change the the Back N Black theme of you choose, either by using the Theme change box on the left on the front page, or by going to your profile.
On the above toolbar, click on "Profile", then "Look and Layout". It is the first option.
One of the compelling reasons for me to make the software change was so people could have a light theme. Due to the age and modifications on the old board, this was impossible.
Please PM or
email me if you have any questions.