Fix source code viewer background color when using Dark Mode in Diffusion

This CSS fix replaces the hard coded white background value by the diff.background CSS variable. This is a proposal
to address this issue but it may be a good idea to create a different CSS variable (for instance source.background)
in order to avoid any potential side effect in the future.

| Before | After |

Fix : T15056

Test Plan:
- Flush all Phorge caches
- Sign in
- Open…
Repository: rP Phorge
Author: bob
Commit Date: Aug 17 2023