I've installed the following theme. It's made for 9700. The installation is by copying files to media card and open jad file in file browser to trigger installation.

Precision ZEN Walk Around Theme for 9700 FREE - BlackBerry Forums at CrackBerry.com

I've rebooted the device but it's not showing up.

Will the theme work for 9300? Or am I missing some steps?

Thanks!

Themes are generally device and OS specific. I doubt that a theme for a 9700 will work on a 9300, since the devices are different.