Fix dark mode for workboard card edit button background
Summary:
Replace hardcoded white background color for the Edit button of a project workboard card (shown when hovering over the card) with variable `{$page.content}` as `CelerityDefaultPostprocessor.php` also defines `'page.content' => '#fff'`.
Refs T15056
Test Plan:
* Go http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a workboard of a project and hover over a card in a column
* Look at the Edit button in the upper right corner of the card
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/D26143