Fix dark mode for Setup Issues
Summary:
Replace hardcoded colors with corresponding variables (so colors can be adjusted accordingly when setting dark mode).
Note that this includes a small color change: `.setup-issue-actions` changed from hardcoded `#f5f5f5;` to very similar `#f3f5f7` (which is `{$page.background}`)
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Enable Prototype applications via `./bin/config set phabricator.show-prototypes true`
* Install UIExamples via http://phorge.localhost/applications/view/PhabricatorUIExamplesApplication/
* Go to http://phorge.localhost/uiexample/view/PhabricatorSetupIssueUIExample/ and look at things
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/D26158