I guess you need to copy the CSS code from the "Customise" page, paste it in a .css file in your mac, change it, copy the code and past it back to the "Customise" page.
But if you mean "On the fly", I do not think it is possible.
TextWrangler is good for editing your CSS. Just copy it from the customize page on your tumblr and paste it into a new document inTextWrangler. It'll automatically parse tags and tabs and make it look pretty and easier to edit.
Thanks guys, a month ago I read a thread about some one hacked CSS edit, and allowed them to edit live, I asked how they did it, but they never responded, I'll keep searching.