How can I shut down my blog while editing?

Sometimes I’ll spend a good 3 straight hours editing themes on my blog and changing the stylesheets to find a good look that I like. I do so by activating new themes and checking them out on the live site as well as saving the edited stylesheet and again checking out the site.

Is there a way I can take the site offline to other users besides myself so they don’t have to see the constant changes every few minutes?
Thanks.