Frank this is a general WordPress Question and you should look there at www.wordpress.org
for information as well.
Well, you can search for "WP Maintenance Mode", URL http://bueltge.de/wp-wartungsmodus-plugin/101/
This will present a message that your Site is in maintenance mode
but, as a logged-in admin you can work with the theme and check how it will look
the best way is to have a local server at your PC, look for XAMP at http://www.apachefriends.org
you can install WP at that server, tweak the theme, export it and import that at your website
so you will be able to test and tweak as you like