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

• Flush all Phorge caches
• Sign in
• Open a diffusion repository
• Open any file
• Check that viewer background color is consistent with the theme.