Once you've selected the form you'd like to modify within the Enterprise admin panel, select the Form Display tab. You'll find it at the rightmost side of the available form tabs. 

Once in Form Display, click the Edit button towards the right of the Form Display field. This will bring up the Edit Form Display dialog box. Read down the list of available choices until you come to the heading "Custom Stylesheet." 

 
You'll also see the following notation: "Placing a stylesheet URL in this field enables you to set your own custom styling to this form. When the form snippet is then shared, the form will have the same custom styling wherever it is placed. Please include a fully realized URL, including 'http://'."
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