There is no default font per se. Each object has its own set of properties and when you drop the object on the form all of those properties come with it. You can simply drag the object onto your palette change it to what ever defaults you want and drag it back onto the Library tab. From there a wizard will pop up and allow you to create a new object or overwrite the last one .....so you coudl have many TextField object with different sets of params to do what ever you want (I suggest you create your own library as your modified obejcts will get overwritten when you upgrade your designer if you use the same names).
Hope that helps