so how to Define a Style in outlook 2007 ?

You can create your own style for mail messages as follow:

1. Open outlook.
2. Click the "new mail message" button.
3. Click "format text" menu.
4. Click the Quick Styles button.
5. Select "Save Selection as a New Quick Style".
6. Fill in the name for the new style in the Name box.
7. Click OK.