Custom field CSS class

Hi.

I created a custom text field for ‘Client position’, but it isn’t given a CSS class. So in the slider output I get:

<div class="testimonial-client">
  <div class="testimonial-name">Client name</div>
  <div class="">Client position</div>
  <div class="testimonial-company">Client company</div>
</div>

How do I give the custom field a CSS class?

Thanks.