[TYPO3] how do you modify the default CSS that comes with the standard templates?

[seml at symbulos.com]

Dear friends,

How do you modify the default CSS that comes with the 
standard templates?

For example: if you create a normal text page content, when 
you try to align right using the rte htmlarea, what the rte 
does is to put class="align-right". But where is the 
"align-right" class in order to modify it?

I am not too sure how to do it.

Thanks in advance.

Corrado
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