rP17befe9bca5b

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

Summary:
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 |
|---------|---------|
|{F333617}|{F333618}|

Fix : T15056

Test Plan:
- Flush all Phorge caches
- Sign in
- Open…