Fix template 12

Summary:
Template 12 used BACKGROUND which was changed to _BACKGROUND, which I am
told we want to discourage the use of.

Test Plan:
* Open template in graphie-to-png
* See it looks the same as before

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D8243