Fix Dark mode for Form inset background color
Summary:
Replace hardcoded background color for form insets with a variable, e.g. when editing a Herald rule.
Split from D25724 written by mturdus.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to e.g. http://phorge.localhost/herald/edit/?content_type=HeraldManiphestTaskAdapter&rule_type=personal
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/D26342