Fix dark mode for Differential revision history selector
Summary:
Replace hardcoded colors of `differential-update-history-*` with variables.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set Accessibility to Dark Mode (Experimental)
* Have a patch `D1` in Differential with several revisions
* On http://phorge.localhost/D1, go to the "Revision Contents" box and select the "History" tab, look at the diff selector table on the right
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/D26212