Improve Dark Mode for large Task Graph Standalone box
Summary:
Use `{$lightyellowbackground}` instead of `{$lightyellow}` for the "This task is connected to more than 200 other tasks" box so the background color is less screaming.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a Maniphest task which is connected to more than 200 other tasks and look at the Task Graph box
Reviewers: O1 Blessed Committers, mainframe98, valerio.bozzolan
Reviewed By: O1 Blessed Committers, mainframe98, valerio.bozzolan
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26385