Theming confusion

Theming confusion

Hi

After reading the docs about theming I am still a little confused.

The basic point seems to be that look and feel classes are in theme.css which can be customised. And that any one widget gets its own css e.g. accordion.css. OK so far.

But:

i)  The draggable widget does not appear to have a plugin specific CSS file based on my custom download. Is this correct?
ii) What is core.css? It is imported by base.css and provides structural markup for helpers. It would be imported if I used all.css but is it required?

Thanks

Justin