Enable user to select their favorite theme or alternative theme while visiting your blog.
There are two kinds of switchable theme. First, you have two themes under your wp-content/themes directory with the same layout ! Then, you can choose in your admin these themes.
Second, you have one theme and you want to switch only few elements in your current theme. To do this, you can make an alternative theme. Create a directory altThemes in your theme directory and a subdirectory for each alternative theme. Keep in mind that the name of the directory will be the name of the subtheme. Finally, create a style.css under your alternative theme and place your css rules insides.
Do not forget that if the rules you want to switch are also defined in mainTheme/style.css, you may need to add the !important rule at the end of your alternative css rules.