[Typo3] where is external css gets included in MTB/1?

[Bing Du]

In section 1.3.6 (Creating the menu), it talks about how changing the 
stylesheet can affect the look and feel of the menu.  But how the 
external stylesheet got included? In the previous sections, I did not 
see how to tell typo3 where to find external stylesheet.

Thanks in advance for any help,

Bing