Setting up Prose behind the scenes

[Future Expat]

This is not necessarily a Prose-specific question, so please move it to the appropriate spot if this isn't it.

I would welcome advice on the best way to redesign my blog behind the scenes. In other words, I'd like visitors to see the old version until I'm ready to unveil the new one. However -- if Prose is not the active theme, I can't make any design modifications. I am a competent WordPress user but absolutely not a programmer. I know html and css, I can recognize PHP enough to cut and paste in the right place, and that's about it.

The blog is fairly robust at this point, with about 200 posts, 20 pages, about 200 images, some video, etc., etc.

Thanks much,
Susanna