I'm a native spanish speaker and both forms have the same meaning "nueva teoría" y "teoría nueva"
http://spanish.about.com/cs/grammar/a/whereadjective.htm http://spanish.about.com/od/adjectives/a/adjectives_word_order.htm http://spanish.about.com/od/adjectives/a/adjective_placement.htm http://spanish.about.com/od/adjectives/a/adjectives_in_spanish.htm http://spanish.about.com/od/adjectives/a/adjectives_quantity.htm
Could you say "Ellos tienen una teoría nueva" and would it still mean "They have a new theory"? Would the meaning be slightly altered?
I think when the adjective is before, in this case, that it means a WHOLE new theory, while "teoría nueva" can be an already existing theory that has been changed somehow.