Should you wish to change the values of any of the CSS classes defined in the first Additional CSS Definitions box, you can do so by copying the class definition from the first box, pasting it into the second box and making the appropriate changes there.
As the contents of the second box appears after the contents of the first box in the final style sheet, any classes defined in the first box and redefined in the second will take their final values from the second box.
For example, the first box contains a definition for .bigusername, setting the font size to 14pt. By redefining the font size for .bigusername in the second box, the definition made in the first box is overridden.

