Do not select any object. Set the attributes you want as the default, and that will be the default from then on. (If you add a field, for example, and immediately change its attributes while it is still selected, that becomes the new default.)
Please note that it's very easy to change default attributes in FileMaker when you don't want this to happen. Many developers set up a layout with fields already formatted as they want and then they first copy and paste a field from there, then double click it to pop up specify fields to specify the desired field for it.