You can control themes of your site from administration menu - and test themes in some course that is hidden from students if you allow course themes.
I said before that you should not use ThemeSwitcher of ThemeRoller on production sites - you can take a copy of Themeroller theme and test with one theme and use another without theme switcher.
It does not actualy matter if students or guests can change styles of theme - nothing changes on server side, the settings are saved only in cookie and when users close the browser or session is deleted they see the original theme again.
A better theme switcher system is to use session themes of moodle itself (see theme docs again... ) and some of the "ready themes" - and when your custom theme is ready just drop the themeroller switcher code and your theme can be used on production site.
Themes
jQuery themes with ThemeRoller
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