Improve Dark Mode for Dependency Graph lines
Summary:
Set 50% transparency on hardcoded white stroke style color (and default filling color) so things look less horrible in Dark Mode and still look fine in Default Mode.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to a task with parent tasks and subtasks, look at the vertical lines in the Task Graph box.
* Probably test the same in Diffusion or Differential but I am too lazy.
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/D26390