Well, we have just got some news for you. Sad news.
BlackBerry did not launch the Theme Builder for OS7 officially because they didnt want any themes for these devices.
Whats more, they will NEVER be releasing a Theme Building for BB10, heck the device wont even have an option.
Farewell Themes on BlackBerries.
I got used to the fact that we wont get themes on OS7, and i wanted to make my own.
So i stuck using a P;9981 theme for a while, then i shifted back to stock evensince i updated to 7.1 (long time back!)
Here is what you can do,