There is not a dev/sandpit area, but the CSS editor does have a Preview button so you can test the changes before they go live. Look for the preview button at Appearance → Custom Design → CSS.

You could make an exact copy of the site as a testing area, but you would have to set it up exactly the same two times and also purchase whatever upgrades you're using on the 2nd blog. If the changes will be extensive and you want them implemented all at once, you might ask the person you're working with to see if they are willing to set up a testing environment by creating a separate test blog on WordPress.com with the same theme. You would have to copy over settings manually, but you can export/import the content from the Tools section in the admin pages. How far you go down this path probably really depends on the extent of the project though.

Can we export the code to another blog and make changes before sending that live?

The custom design upgrade applies only to one blog.

I have flgged this thread so it is moved to the CSS editing Forum for you. There you can get answers and help from those who assist with CSS editing. Best wishes. :)

Not being too techy minded, I picked a standard theme which worked very well, but we now want to expand and customise it. I have bought the CSS pack and now have somebody to code.

So whats the best way to go about doing it?

Do I give the other user Editor (or is it Admin) Access to the site and let them change around? Is there a dev/sandpit/test area where they can make the changes and I can review before we press the button? Can we export the code to another blog and make changes before sending that live?

Any help is very much appreciated