Fix Dark Mode for mini badges below user avatar
Summary:
Do not hardcode white color; use a variable instead so there is sufficient contrast in Dark Mode.
Refs T15056
Test Plan:
* Assign a badge to a user
* Log in as that user
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a task and look at the badge icon below the user avatar.
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/D26406