Small trick: we add a filter to the Django templating in order to add some class to our form inputs

app/app/settings.py

@@ -97,6 +97,7 @@ TEMPLATES = [
             ],
             'libraries': {
                 'analytics': 'server.templatetags.analytics',
+                'utils_templating': 'authentification.templatetags.utils_templating',
             },
         },
     },

app/authentification/templatetags/utils_templating.py

@@ -0,0 +1,10 @@
+from django import template
+
+register = template.Library()
+
+@register.filter(name='addcss')
+def addcss(value, arg):
+    css_classes = value.field.widget.attrs.get('class', '').split(' ')
+    if css_classes and arg not in css_classes:
+        css_classes = '%s %s' % (css_classes, arg)
+    return value.as_widget(attrs={'class': css_classes})