I was briefly very confused until I figured out you were correcting Themes to Themas, and the dropped "l" was just an accident!
If you were only briefly confused, I congratulate you - it would have confused the heck out of me for much longer.
> Metamagica Themas
Metamagical Themas
(You left out the L.)
It's Metamagica Themas, I think - an anagram of 'Mathematical Games'; the Martin Gardner's column in Scientific American which proceeded it.
A lovely book.