Fix dark mode for Badge large view
Summary:
Replace hardcoded white background color for a Badge with variable `{$page.content}` as `CelerityDefaultPostprocessor.php` also defines `'page.content' => '#fff'`.
Note that fixing the surrounding background color is a separate topic as that uses a common variable.
Refs T15056
Test Plan:
* Go http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Create a badge; go to http://phorge.localhost/badges/view/1/
* Assign badge to a user, go to http://phorge.localhost/people/badges/1/
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26145