The default Style name is Style 1.
 To change the default Style:
1. Select Home -> Cell Styles
(in Styles Group) -> New
Cell Style.
2. In Style dialog box click
Format.
3. Select the Number tab.
4. In the Category box, select
Custom.
5. In the Type box, enter the
following format (or change
to any desired number formatting):
#,##0;[Red]-#,##0;0;@ (or any other custom number formatting chosen)
6. Select the Font tab.
7. Select a font and the desired font size, and then click OK.
Note: This only changes the Style1 style for the current workbook.