Restore white background to Paste

Summary: I think we lost this recently (maybe with the line highlighting change), but it's more readable if we put these on a white background.

Test Plan: Looked at a Paste (like `/P123`), saw white background.

Reviewers: chad, btrahan, asherkin

Reviewed By: asherkin

CC: aran

Differential Revision: https://secure.phabricator.com/D6852