Fix dark mode for Differential Revision size indicators
Summary:
Introduce a bunch of variables instead of hardcoding colors.
Refs T15056
Test Plan:
1. Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
2. Have some open Revisions in Differential with different sizes, go to http://phorge.localhost/differential/query/all/
Reviewers: O1 Blessed Committers, amybones, mainframe98
Reviewed By: O1 Blessed Committers, amybones, mainframe98
Subscribers: amybones, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26197