The old theme is no longer there. We have the default Theme (with the logo at the top, which is what we call "responsive" so it can be used on tablets, and the "Core" theme, which sort of looks squarish.
The core theme has not been modified for our site, the default (Curve) theme has.
To change themes go to Profile / Modify Profile / Look and Layout / Current Theme (change)
But as Benzo said... you don't want to go there. I may in fact delete it.
Peter