Donncha O Caoimh (a11n)

This plugin allows a blog administrator to test a new theme while showing the old theme to regular visitors.

Theme Tester

Test a theme on your blog without showing your visitors

Federico Jacobi

An extension for the Theme to Browser Control (T2B) to handle different versions of IE. Requires the T2B plugin to be installed first https://wordpress.org/plugins/theme-to-browser-t2b-control/.
Adds IE versions from 6 to 11 to be handled separately.
PLEASE vote and/or rate if it works for you or let me know if there’s a fix needed at web[at]federicojacobi.com.

Theme to Browser Control – IE Pack

Displays different themes based on the browser used. Extension for Theme-to-Browser plugin

Theme Tester vs Theme to Browser Control – IE Pack