Hidden theme preview

Hi,

Is there any way I can design a theme and preview it using some kind of address so only I can see the new version (For previewing/testing) yet the rest of the world can see the current style.

For example, could I enter something along the lines of http://www.jasonmhirst.co.uk/theme=newtheme or similar

It’s a bit of a pain swapping from one theme to another all the time.

TIA