Fix dark mode for Differential Keyboard Reference dialog
Summary:
Replace hardcoded background color `#ffffff` for items in the Keyboard Shortcuts navigation panel of Differential with variable `{$page.content}`.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Create a Differential revision
* Go to http://phorge.localhost/D1
* In the left upper corner, click that arrow thingie to Show Paths Panel
* At the bottom of that overlay panel, select "Keyboard Reference"
* Look at the navigation panel of the "Keyboard Shortcuts" overlay dialog
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26163