Fix dark mode for workboard trigger preview dialog
Summary:
Do not hardcode the background color for the `workboard-drop-preview` but use the {$page.content} variable instead.
Refs T15056
Test Plan:
* Go http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a project workboard with at least one non-default column which is not a proxy
* Set up a workboard trigger via the cogs button in the header of a workboard column
* Hover over the cogs button and look at the preview displayed in the bottom right corner
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/D26139