if it is a theme the the format should xap
how to opened a xap file?
follow these steps : Requirements:
A developer unlocked device. To unlock your device, follow the guide here.
“Windows Phone 7 Developers Tools” downloaded and installed on your computer. Grab it from here
Zune installed and working on your PC.
The XAP file you want to deploy stored to a known place in your PC.Instructions:
From your computer go to the Start menu and navigate to “Windows Phone Developer Tools”.
From among the options look for “Application Deployment” and select it.
When asked to specify a target device for deployment, choose “Windows Phone 7 Device”.
Direct the tool to the file path of the XAP entity you want in your phone.
Plug in your phone to the PC via USB cable.
Let Zune sync with your device.
Without letting your phone go to sleep, press the Deploy button in the tool running on your PC.
You don’t even need to restart your phone, the app will be visible in your phone’s app list after some time.
Hope it work and helpful
Ps : before trying the steps above, it should better if you try the steps mentioned here first : http://wp.pandaapp.com/news/11042011/002032112.shtml