Although I changed my vote to YES to allow WotC themes once they were in Character Builder and easy to access, I will definitely be voting NO to allow player created themes.
Themes are a bad idea to begin with. They are a souped up version of Backgrounds (i.e. free abilities).
I do not know how many PCs with the Ritual Caster feat that I have played that did not get the Alchemist feat as a class feature and I had to think hard about whether that PC wanted to spend a feat on Alchemist to round out the PC's item creating capabilities. Now, a player can just go pick up Alchemist and bang: free Alchemist feat. Yes, that PC doesn't have some other icing on the cake theme, but it effectively gives those PCs that wanted Alchemist a way to gain a freed up feat slot.
Last summer, WotC upped the damage of monsters. With the introduction of themes, they are yet again giving more power and abilities to the PCs. It becomes an arms race yet again (like previous editions).
When the game has a set number of abilities that each PC can get, yes, there is power creep as more and more abilities are introduced. But at least the PCs are not becoming more complex due to more abilities on the character sheet. The power creep is even more substantial when the game outright hands out more abilities per PC.
And as I've seen on Encounters nights and on Game Days, even low level PCs have quite a few abilities and it's tough for even experienced players to handle all of the options of a class that the player hasn't played before, let alone new players. As more and more creep gets added into the game system and free abilities are handed out to PCs, the PCs get even that much more difficult to play well because they start acquiring a plethora of abilities.
I dislike themes, but as long as it is official WotC material and can easily be added to a PC with Character Builder, I'm kind of begrudgingly ok with it. But I really don't like the concept of adding a bunch of house rule themes.