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 a diffusion repository
- Open any file
- Check that viewer background color is consistent with the theme.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25396

resources/celerity/map.php

@@ -121,7 +121,7 @@ return array(
     'rsrc/css/fuel/fuel-handle-list.css' => '2c4cbeca',
     'rsrc/css/fuel/fuel-map.css' => 'd6e31510',
     'rsrc/css/fuel/fuel-menu.css' => '21f5d199',
-    'rsrc/css/layout/phabricator-source-code-view.css' => '49656486',
+    'rsrc/css/layout/phabricator-source-code-view.css' => 'e382316a',
     'rsrc/css/phui/button/phui-button-bar.css' => 'a4aa75c4',
     'rsrc/css/phui/button/phui-button-simple.css' => '1ff278aa',
     'rsrc/css/phui/button/phui-button.css' => 'f9d0f9c8',
@@ -805,7 +805,7 @@ return array(
     'phabricator-search-results-css' => '9ea70ace',
     'phabricator-shaped-request' => '995f5102',
     'phabricator-slowvote-css' => '1694baed',
-    'phabricator-source-code-view-css' => '49656486',
+    'phabricator-source-code-view-css' => 'e382316a',
     'phabricator-standard-page-view' => 'e08c7462',
     'phabricator-textareautils' => 'f340a484',
     'phabricator-title' => '43bc9360',

webroot/rsrc/css/layout/phabricator-source-code-view.css

@@ -16,7 +16,7 @@
   white-space: pre-wrap;
   padding: 2px 8px 1px;
   width: 100%;
-  background: #ffffff;
+  background: {$diff.background};
 }
 
 .phabricator-source-line {