How to style WTForm fields using custom css classes?

Post
 

{{ form.submit }}

# renders

[type="submit"] {
  color: #000000;
  font-size: 16px;
  border: 1px solid #e5e5e5;
  padding: 10px 25px;
  display: inline-block;
  background-color: #d7d7d7;
  font-weight: 600;
  cursor: pointer;
}