Does cisco have a zipped pack of template guest html and css files that you can modify to customize the portals for both guest and sponsor?
you can customize your portals with https://isepb.cisco.com/
Beyond the ISE Portal Builder that Vincenzo mentioned, you can download the source CSS files directly from your ISE PAN via the GUI.
The exact location varies according to the release, but the Admin Guide will give details. for instance, here is how in the current latest version (ISE 2.1):
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