Improve Dark Mode for diffs
Summary:
Use colors which somehow work, though likely improving all the "Codeblock syntax highlighting" variables for Dark Mode may also make sense at some point.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Look at text file differences in a revision diff like http://phorge.localhost/D1
* Edit the text description of a task and click "(Show Details)" in the timeline entry
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/D26401