The theme only provides you with a place to put your email address. This is programmed to initialize the “mailto” agent as setup in your browser. In most browers, you should get a popup similar to this one (Firefox)
This lets you choose from all the email options found on your computer. If you check the Remember box, all mailto links will open using this method in the future.
Check your browser’s settings for how to manually change how mailto links if you do not get this popup.