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.
- 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

resources/celerity/map.php

@@ -160,7 +160,7 @@ return array(
     'rsrc/css/phui/phui-head-thing.css' => 'd7f293df',
     'rsrc/css/phui/phui-header-view.css' => '36c86a58',
     'rsrc/css/phui/phui-hovercard.css' => '39fd2e14',
-    'rsrc/css/phui/phui-icon-set-selector.css' => '7aa5f3ec',
+    'rsrc/css/phui/phui-icon-set-selector.css' => '19e0253b',
     'rsrc/css/phui/phui-icon.css' => '084ac612',
     'rsrc/css/phui/phui-image-mask.css' => '62c7f4d2',
     'rsrc/css/phui/phui-info-view.css' => 'a10a909b',
@@ -856,7 +856,7 @@ return array(
     'phui-hovercard' => '6199f752',
     'phui-hovercard-list' => 'de4b4919',
     'phui-hovercard-view-css' => '39fd2e14',
-    'phui-icon-set-selector-css' => '7aa5f3ec',
+    'phui-icon-set-selector-css' => '19e0253b',
     'phui-icon-view-css' => '084ac612',
     'phui-image-mask-css' => '62c7f4d2',
     'phui-info-view-css' => 'a10a909b',

webroot/rsrc/css/phui/phui-icon-set-selector.css

@@ -3,8 +3,8 @@
  */
 
 button.icon-button {
-  background-color: #F7F7F9;
-  background-image: linear-gradient(to bottom, #ffffff, #f1f0f1);
+  background-image: linear-gradient(to bottom,
+    {$lightgreybackground}, {$greybackground});
   border: 1px solid rgba({$alphablue},.2);
   color: {$darkgreytext};
   position: relative;