making listbox.insert item bold

in my code-behind after all the items are inserted I want to insert an item at the 0 index position.  That item has to be bold.  How would I do this?

 This is what I have so far:
Label lblListBoxCriteria = new Label();
lblListBoxCriteria.Text = "Change Criteria";

lblListBoxCriteria.CssClass = "listItemCriteria";

lstCriteria.Items.Insert(0, lblListBoxCriteria.Text);

But the item doesn't appear bold. 

Any suggestions? Thanks.
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