I don't think updating the CSS in your epubs en-masse is as simple as this.
For instance if you put
Code:
h1 {text-align:center;}
in ExtraCSS but the HTML in your epub is something like
Code:
<h1 class="mystyle">Chapter One</h1>
and existing CSS in the epub something like
Code:
.mystyle {text-align:left}
then your ExtraCSS won't override what's already in the epub.
You would need to be more specific in ExtraCSS e.g.
Code:
h1.mystyle {text-align:center}