Set background of radio container as transparent

Summary:
Before:
https://s3.amazonaws.com/uploads.hipchat.com/6574/690355/TzDPWkVuDkTd8JH/before.png

Note the white background between the radio options above.

After:
https://s3.amazonaws.com/uploads.hipchat.com/6574/690355/flxevn250QklQJT/after.png

If this looks like a step backward, realize that it will look right
when combined with D12202.

Test Plan:
Load "Count to 100".
Enable the scratchpad.
Assert that the background color for the radio widget is transparent.

Reviewers: aria

Reviewed By: aria

Subscribers: kasra

Differential Revision: http://phabricator.khanacademy.org/D12207