Thank you for your question. Unfortunately no, you can’t customize embeded Signature cloud service header. All available settings for the Signature service which you can use is in your GroupDocs account settings .
In case using Viewer service (sorry form your post is not clear which GroupDocs service you will use Viewer or Signature) you can customize header of the embeded Viewer, you can change settings in the same place as for the Signature service but on the “Viewer” tab .
Thank you for clarification. Unfortunately no, the settings for the header which you have for the Viewer will not be applied for the Signature because it’s absolutely different services (if you will check URL which you have in the iframe you will see that).
Is it possible to auto expand the sign box and remove the choose color options so the actual signature window is larger or ideally sizes to the size of the device?