Fix dark mode for date picker
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