If you want to use one theme for a Magento website when it is loaded via a personal computer and a different one for mobile devices follow the below steps:
At first, select the preferred mobile Magento theme and install it as shown in our tutorial here.
Don’t type its name in the Default theme field. In this way the default theme will be loaded when the website is browsed through a computer.
Then add exceptions for the mobile devices. Go to the Magento administrative area -> System -> Configuration -> Design -> Themes. After this, click on the Add Exceptions buttons near the Templates, Skin and Layout labels.
In the Matched Expression field, enter the below line:
iPhone|iPod|BlackBerry|Palm|Googlebot-Mobile|Mobile|mobile|mobi|Windows Mobile|Safari Mobile|Android|Opera Mini
In the Value field type the name of your newly installed mobile theme and click on Save.
Now, you can open your website via your computer and your mobile phone. You will see that it loads with two different themes based on the device.