Restore an explicit white background color to files in Paste

Summary:
Ref T13105. Previously, the "source code" view in Paste rendered on a brown/orange-ish background. I've been using this element in more contexts (Files, Diffusion) and removed the colored background to make text (particularly syntax-highlighted text) easier to read and reduce visual noise with the new blame colors.

In Diffusion the view is in a box with a white background so removing the background left us with white, but in Paste it's just directly on the page so the background was bleeding through. Instead, set it to white explicitly.

Test Plan: Viewed source files in Files, Diffusion and Paste; saw text on a white background.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19346