Merge forwardport of #12931 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12931.patch (created by @jalogut) based on commit(s):
  1. c66fde6

Author: magento-engcom-team

app/design/adminhtml/Magento/backend/Magento_Backend/web/css/source/module/main/actions-bar/_store-switcher.less

@@ -42,6 +42,14 @@
         max-height: 250px;
         overflow-y: auto;
         padding-top: .25em;
+        &::-webkit-scrollbar {
+            -webkit-appearance: none;
+            width: 7px;
+        }
+        &::-webkit-scrollbar-thumb {
+            border-radius: 4px;
+            background-color: rgba(0, 0, 0, .5);
+        }
 
         li {
             border: 0;