How to edit the “inactive” theme while using Mobile Smart?

I was thinking of using this plugin so I can keep my old non-responsive theme active for desktop and then using a mobile-friendy theme for mobile visitors only. Would doing this cause any issues with cache/minify plugins? Secondly, since the mobile-friendy theme would remain ‘inactive’ in WordPress, I won’t be able to edit it in the “Appearance -> Customise” screen. I tried using some Theme Test Drive plugins to edit it but they didn’t allow me to save the changes. Is there any way at all to customise inactive themes and save the changes without having to activate/publish the theme?