Fix dark mode for Conpherence Persistent Chat menu
Summary:
Replace hardcoded `background` color with variable `{$page.content}` because `.phuix-dropdown-menu` also defines `background: {$page.content}`.
Refs T15056
Test Plan:
* Go http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Join some Conpherence room
* In the top bar, click the speech bubbles icon and enable "Persistent Chat"
* In the Conpherence overlay in the bottom right corner, click the "cogs" button and see the menu
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/D26137