Those sign-up forms, while they do accept some additional styling, for the most part the changes you are looking to make may be a bit more than what our base forms are designed for. You may benefit from creating your own custom sign-up form using out API.
By creating your own custom sign-up form you can create the form to accept pretty much any styling and/or look you want. If you continue to use the forms we provide you will be able to adjust some options by adding/adjusting the CSS on your website, but not everything.
If you want to create your own sign-up form using our API you can find our documentation located here: https://developer.constantcontact.com/docs/developer-guides/overview-of-api-endpoints.html
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