Research In Motion has been relatively dragging their feet in support of BlackBerry themes. It isn’t a high priority for them as BlackBerry 10 will not have theme capabilities.
Nevertheless, theme developers looking to make themes for OS 7+ are getting a little advance treatment. BlackBerry Theme Builder 7.1 Beta has leaked online.
BlackBerry Theme Builder 7.1 supports most of the OS 7 devices out there, which include models; 9981, 9930/9900, 9860/9850, 9810, 9790, 9380, 9360/9350, 9320/9220, and 9310. You can also export the theme for OS 7.0 or OS 7.1 compatibility.
Remember, this is beta and likely full of bugs. However, it will nevertheless allow you at accomplishing theme development for the aforementioned devices.