Improve Dark Mode in global Notifications dropdown
Summary:
Replace hardcoded black color for values in the notifications dropdown with a variable in `.phabricator-notification .phui-timeline-value`.
Refs T15056
Test Plan: Open top bar Notification menu in Dark Mode (text now readable) and in Default Mode (see slightly lighter text but still in italic for differentiation).
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26382