Improve Dark Mode for borders of PHUIBoxView and Setup issues
Summary:
Do not hardcode the border of some boxes to `#e2e2e2` which is unnecessarily bright in Dark Mode but set the `{$lightgreyborder}` variable instead which corresponds to `#C7CCD9` in Default Mode - close enough.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to http://phorge.localhost/settings/panel/apitokens/ and look at the box
* Go to http://phorge.localhost/config/issue/ and open one of the listed issues
* Scroll down on one of the listed issues and check the "To continue, resolve this problem and reload the page." box
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: amybones, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26232