How do I keep my current concrete5 site live while I work on restructuring my site with a new theme?

I need to customise the new theme that I have installed, change/add navigation titles and change/add new content to current pages without affecting the current live site.

Is there a way to do this in concrete5 without putting the site in maintenance mode? When I put the site on maintenance mode in Systems & Settings, I can't preview the new theme without switching maintenance mode on and off to view the changes.

Would appreciate any help/ideas!