Add white background to Perseus radio options

Summary:
Before:
https://s3.amazonaws.com/uploads.hipchat.com/6574/690355/GRrNTgFBuJ3J1X1/before.png

After:
https://s3.amazonaws.com/uploads.hipchat.com/6574/690355/B8qCR1VxAeXrlBz/after.png

Test Plan:
Load "Structure in expressions 1".
Enable the scratchpad.
Assert that the background color for the radio options remains white.

Reviewers: aria

Reviewed By: aria

Subscribers: kasra

Differential Revision: http://phabricator.khanacademy.org/D12202