How highlight problem fields?

I'm using Ubercart and would like to highlight some fields in the cart if user doesn't have required quantity in the cart. I check it in hook_form_alter for uc_cart_view_form, however I've got no idea how to apply class to the product quantity field ($item['qty']) in order to use theme_textfield to spit out error class. Perhaps, I'm lost, leas help me.

P.S. I see that theme_textfield doesn't give a damn about which class I apply, it is still "form-text". What should I do?