Improve Dark mode on workboards for empty columns and milestone columns
Summary:
Replace hardcoded background colors for empty columns and milestone columns on project workboards.
Split from D25724 written by mturdus.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to http://phorge.localhost/project/board/1/, create a column with no task cards and look at its background color
* Go to http://phorge.localhost/project/board/1/, create a milestone in that project and look at its column color on the board
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/D26335