This page consists of screenshots from the Subscription Form Design – Form Buttons Screen in Coming Soon Booster WordPress Plugin.
This gives an idea about how to configure settings for Submit Button of Subscription Form. You can customize the text which has to be displayed on the submit button.
You can customize the Button Color, Border Color, Font, Alignment, Margin and Padding for the Subscription Button. You can also change Button Color and Border Color on mouse hover.
Using Live Preview, you can get an idea about how Button would be viewed on Subscription Form.