Improve Dark Mode for gentle highlight box
Summary:
Use a more "gentle" color in dark mode to make text more readable.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a Maniphest task _again_ which has many comments and activity and look at the "There are a very large number of changes, so older changes are hidden. Show Older Changes" box color
* Go to a Differential revision which includes binary files and look at the "This is a binary file" box color
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/D26386