You can change the site theme from the web.config file. Look for the pages tag - it should look like this:
<pages theme="Default" enableEventValidation="false" validateRequest="false" enableViewStateMac="false">
Notice the theme attribute. You can change the value from "Default" to whatever theme you would like. For example if you would like to switch to the "Adult1" theme then you should change the line like this:
<pages theme="Adult1" enableEventValidation="false" validateRequest="false" enableViewStateMac="false">