R1:3f5fcdf4ddd8

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.

| Before | After |
|---------|---------|
|{F331622}|{F331623}|

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.