R1:55619e89642b

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…