Fix dark mode for date picker master
Summary: Replace the hardcoded background color of `.fancy-datepicker-core` with the same background color that `.phui-box-border` uses: `{$page.content}`. Refs T15056 Test Plan: 1. Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)` 2. Run `./bin/config set phabricator.show-prototypes true` 3. Install Calendar at http://phorge.localhost/applications/view/PhabricatorCalendarApplication/ 4. Go to http://phorge.localhost/calendar/ 5. Select "Create Event" in upper right corner 6. Click the small date picker button for Start or End fields Reviewers: O1 Blessed Committers, valerio.bozzolan Reviewed By: O1 Blessed Committers, valerio.bozzolan Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15056 Differential Revision: https://we.phorge.it/D26188