Fix Dark Mode for Conpherence chatroom list background colors
Summary:
Fix some light Base Colors to be darker in Dark Mode. They had been the same as in Default Mode so far.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to Conpherence room `Z1`
* Click the Settings cog button in upper right corner
* In "Room Preferences", set "Theme" to "Pink" or to "Indigo"
* Look at the sidebar listing all chatrooms
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/D26391