Issue #2918080 by maxk: Logo image not responsive

templates/block/block--system-branding-block.html.twig

@@ -15,7 +15,7 @@
 {% set attributes = attributes.addClass('site-branding') %}
   {% if site_logo %}
     <a href="{{ path('<front>') }}" title="{{ 'Home'|t }}" rel="home" class="float-left site-logo">
-      <img src="{{ site_logo }}" alt="{{ 'Home'|t }}" />
+      <img src="{{ site_logo }}" alt="{{ 'Home'|t }}" class="img-fluid" />
     </a>
   {% endif %}
   {% if site_name or site_slogan %}