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/suggest
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.orgyou 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