I am user of Nokia N8 and I also want to know the technique to create my own theme for my n8 and my search resulted in finding this site www11.ownskin.com which allows registered user to download or make their own themes. its an online theme creator which supports Nokia Symbian S60, S40 and Sony Ericsson and many other models. You can download the theme for the phone and install on your phone.
Below steps to make your own Nokia Theme that has to be followed:-
>>Go to OwnSkin and create a username for you. Signup and then login.
>>Select The type of skin, which will be according to the type of Nokia Phone you have.
>>Select an Image and crop to a particular size.
>>Select an Image for the active background, which will show in the menu item.
>>Go to advance settings to change the contrast of background, choose another image for the clock and battery.
>>Enter the information about the skin and choose if you want to share it or keep it private.
>>Download and Install the skin.
Hope it will help you.