gnome-shell: Style login/unlock default user icon

Bug: vertical user icon color in the unlock screen for the Arc-Lighter
variant is incorrect (dark color) due to upstream limitations on
separate styling for login and unlock screens

#72

Author: drakkar1969

common/gnome-shell/3.36/sass/_common.scss

@@ -2428,15 +2428,20 @@ StScrollBar {
 //
 .user-icon {
   background-size: contain;
-  color: $fg_color;
+  color: $osd_fg_color;
   border-radius: 99px;
+
+  .login-dialog-user-list-item:selected & { color: $selected_fg_color; }
+
   &:hover {
-    color: transparentize($fg_color, 0.3);
+    color: $osd_fg_color;
   }
 
   & StIcon {
-    background-color: transparentize($osd_fg_color, 0.95);
+    background-color: transparentize($osd_fg_color, 0.75);
     border-radius: 99px;
+
+    .login-dialog-user-list-item:selected & { background-color: transparentize($selected_fg_color, 0.75); }
   }
 }