I don’t know about changing the default, but you can save a style and assign it to a function key. For example we use Arial 26 bold with a 3pt shadow every day, so I saved that style and assigned it to F2, so when I open the title tool and press F2 I’m ready to go. May not be exactly what you’re looking for, but might be close.
After you create and save a style in the Title Tool, it will show up under the Settings tab of the Project window. Double click the style name in the Project window, and enter the function key you want to assign it to in the Function Key box. That’s it.