I searched a bit and @markstory said "I don't think themes have historically had helpers as part of the documented features." that was in 2011.
There is no theme loader in lib/Cake/View/HelperCollection.php load() function. 

Here is a hot fix possibility from @majna https://github.com/majna/cakephp/commit/ac91ffaaff02ff1ea881e0d6dcd9e2140e191243