Accessing Non-Current Theme

Install the theme-switcher plugin and activate it.
this plugin is available at https://dev.wp-plugins.org/wiki/PluginDirectory

then you go to https://davidyeiser.com/?wptheme=yournewthemename

thats it. then only you will be seeing the site in this theme, while all others view the site, with the currently active theme.

I do this all the time. hope that helps.
Sadish

Excellent, this works. Although I had to use https://davidyeiser.com/index.php?wptheme=yournewthemename to get it to work. Thanks very much!