Show datetime in flash notices in white, if background is dark.

This highly increases readability of the datetime for most flashes.

templates/_flash.html.twig

@@ -17,13 +17,13 @@
                     {# <img src="..." class="rounded mr-2" alt="...">#}
                     <i class="fas fa-fw {{ flash_symbol }} mr-2"></i>
                     <strong class="mr-auto">{{ flash_title|trans }}</strong>
-                    <small class="text-muted">{{ "now" | format_datetime("short", "short") }}</small>
+                    <small class="{% if "text-white" in flash_bg %}text-white{% else %}text-muted{% endif %}">{{ "now" | format_datetime("short", "short") }}</small>
                     <button type="button" class="ml-2 mb-1 close" data-dismiss="toast" aria-label="Close">
                         <span aria-hidden="true">&times;</span>
                     </button>
                 </div>
                 <div class="toast-body {{ flash_bg }}">
-                    {{ message | trans}}
+                    {{ message | trans }}
                 </div>
             </div>
         {% endfor %}