Merge forwardport of #12862 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12862.patch (created by @jonshipman) based on commit(s):
  1. 862e74a
  2. 7c02acb

app/code/Magento/Theme/Block/Html/Topmenu.php

@@ -235,7 +235,13 @@ protected function _getHtml(
 
             if ($childLevel == 0 && $outermostClass) {
                 $outermostClassCode = ' class="' . $outermostClass . '" ';
-                $child->setClass($outermostClass);
+                $currentClass = $child->getClass();
+
+                if (empty($currentClass)) {
+                    $child->setClass($outermostClass);
+                } else {
+                    $child->setClass($currentClass . ' ' . $outermostClass);
+                }
             }
 
             if (count($colBrakes) && $colBrakes[$counter]['colbrake']) {