Add to Theme or Start from scratch?

Hello,

When I Edit CSS using :

https://www.ads-software.com/extend/plugins/safecss/

Which option should i select?

– Add this to the Twenty Eleven theme’s CSS stylesheet

or

– Start from scratch and just use this

…what are the consequences of selecting one or the other?
I actually selected one for a few MU sites and the other for another few and can’t see a difference…

Thanks!