Fix dark mode for Outline Tags
Summary:
Replace hardcoded background color `#fff` with variable `{$page.content}` on `.phui-tag-view.phui-tag-type-outline`.
Refs T15056
Test Plan:
* Go 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/PHUITagExample/ and scroll down to the "Outlines" section
* Alternatively, configure some task subtypes at http://phorge.localhost/config/edit/maniphest.subtypes/ and look at the top of a Maniphest task
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/D26160