Below is the simple trick on unlocking new themes in your Windows 7 PC.

Step I -  Go to  My Computer.

Step II - Click on Organize link located on the upper left side of its toolbar and than click on Folder and Search Options as shown below.

Step III - Cick the View Tab and tick Show hidden files, folders and drivers and untick Hide protected operating system files (Recommended) and click on OK as shown below.

Step IV - Now go to the following folder: C:\Windows\Globalization\MCT\

Step V - Under MCT folder you should be able to see five folders having MCT-XX format such as: MTC-AU, MTC-CA,etc. These are international settings for each country as shown below

Step VI - Inside each of the above folder you should see theme folders. Double click theme folder to open it.

Step VII - Now double-click on the .theme file to unlock it as shown below

Step VIII - Once a theme is unlocked it will be available in the Personalization options as shown below