netbox/netbox/templates/inc/notification_bell.html

{% if notifications %}
  <span class="text-primary" id="notifications-alert" hx-swap-oob="true">
    <i class="mdi mdi-bell-ring"></i>
    <span class="badge bg-red"></span>
  </span>
{% else %}
  <span class="text-muted" id="notifications-alert" hx-swap-oob="true">
    <i class="mdi mdi-bell"></i>
  </span>
{% endif %}