How do you make a copy of a Theme for testing?

I tried renaming the theme folder and changing the name used in "theme_description.php" but does not seem to work 100%. Searched but can't find answer.

Forgot to mention, trying to make a copy of ZpArdoise and rename.

I see the duplicate button, but not available for zpArdoise

You copy the folder of the theme and rename it (I never used the buttons and do this straight via FTP). That's all.

Of course changing the theme description also is needed. Also to avoid shared options of the orginal and the copied theme you should rename the options.