Hi Jorge,
The fature that you are after is not available using the standard options of our theme. You need to add custom CSS code and that is outside of our support scope. We will do our best to guide you but we will not be able to implement the feature.
As my colleague mentioned you need to use the absolute positioning on the button element. You do not need any classes and you can use the Element CSS feature. You can read more about the Element CSS feature here:
The code should be something like this:
$el {
position: absolute;
bottom: -20px;
right: 0;
}
The code above is not the working code as we do not know the URL of the page you gave the screenshot. It can be used as a boilerplate for your customization.
Thank you for your understanding.