Fix icon background color when using Dark Mode

Summary:
This CSS fix replaces the hard coded background gradient (white) value of icons when using Dark Mode.

Now the "Choose User Icon" popup has visible icons.

Ref T15056

Test Plan:

• Flush all Phorge caches
• Sign in
• Go to user's Settings > Display Preferences and select the Accessibility (user interface) "Dark Mode".
• Go to user's profile, edit profile and click on Choose icon.
• Check that now the icons in "Choose User Icon" are visible.
• Do these steps for each user interface theme in order to check against regression.

Reviewers: O1 Blessed Committers, Matthew, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Matthew, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15056

Differential Revision: https://we.phorge.it/D25384