Hi Jay,
Here are the basemaps we currently support in Connect, when you set it in Connect, the webform will honor it.
Currently, there is no webform version preview in Connect. Please note that Connect and web designer offer different funtionality and we try to sync the support for both Connect and web designer. You will see one being support and the other does not with certain functionalities. To honor the default basemap in web designer in the field app, we currently have a working salesforce issue open:
BUG-000124917: Default Map location from web designer is not honoured in field app.
Hope this help to clarify your question.
Thanks,