First Install MyTheme plugin.
http://mods.mybb.com/view/mytheme
Then you can add a link anywhere to select a style, by the following code.
Quote:<a href="yourforum.com/index.php?action=mytheme&style=themeIDnumber" >Theme name</a>
Replace Yourforum.com with your forum URL, "
themeIDnumber" with the ID number of the theme (you can find it by hovering over theme name in ACP), and
Theme name with the name of the theme.