To add the button press + and choose Button under Basic Componenets:
Use Properties menu on the right to edit your button. Here you can:
add text to your button
add link or email
Note: To add an email type:
mailto:someone@yoursite.com
change buttons' variant, color and and size
add icons or animations
Use Style menu on the rigth to add padding or margin to your button.
To edit the color of your button press Paint Bucket and choose one of the Website colors, Gradient colors or your color.