We're really sorry, but there is not an easy way for this kind of customization. The button is located within another div. Besides, for non-logged-in users, there are some other fields, so you'll need to customize the name, email etc. fields as well.
In case you want to say thank you! 🙂 We'd really appreciate if you leave a good review on the plugin page. This is the best way to say thank you to this project and the support team.