diff --git a/themes/socialbase/src/Plugin/Preprocess/SocialBaseMenuLocalAction.php b/themes/socialbase/src/Plugin/Preprocess/SocialBaseMenuLocalAction.php
index <HASH>..<HASH> 100644
--- a/themes/socialbase/src/Plugin/Preprocess/SocialBaseMenuLocalAction.php
+++ b/themes/socialbase/src/Plugin/Preprocess/SocialBaseMenuLocalAction.php
@@ -22,7 +22,10 @@ class SocialBaseMenuLocalAction extends MenuLocalAction {
 
     parent::preprocessElement($element, $variables);
 
-    if (\Drupal::routeMatch()->getRouteName() === 'entity.group_content.collection') {
+    // Identify the following buttons:
+    // `Add member` on the manage members page of a group;
+    // `New message` on the private message page.
+    if (\Drupal::routeMatch()->getRouteName() === 'entity.group_content.collection' || \Drupal::routeMatch()->getRouteName() === 'entity.private_message_thread.canonical') {
 
       $variables['link']['#options']['attributes']['class'] = 'btn btn-primary btn-raised';
       $variables['attributes']['class'][] = 'margin-bottom-l';
DS-<I> by maikelkoopman: theme the   button