Go to "Website Customizations" and choose "Customizations" to change the theme.
Choose your design and click on "use". Your theme will be changed right away.
You can try as may templates as you want without losing your customized settings. You can always go back to "Personal Theme". But be careful: If you start changing things, like the fonts or the structure in one of the themes, that one will become your new "Personal Theme" and your former settings will be lost.