Fix transparent avatars showing underlying elements in avatar stack and profile (#10422)

app/javascript/styles/mastodon/components.scss

@@ -5308,6 +5308,7 @@ noscript {
       margin-left: -2px;
 
       .account__avatar {
+        background: darken($ui-base-color, 8%);
         border: 2px solid lighten($ui-base-color, 4%);
       }
     }

app/javascript/styles/mastodon/widgets.scss

@@ -352,6 +352,7 @@
     border-radius: 50%;
     position: relative;
     margin-left: -10px;
+    background: darken($ui-base-color, 8%);
     border: 2px solid $ui-base-color;
 
     &:nth-child(1) {