Export your creation as an .nth file. Once saved, simply select it from your phone's Gallery or Theme menu to apply. π₯ Free Download Link
: Themes created are generally compatible with S40 3rd Edition and newer. How to Create Themes Manually
Transfer the .jar file to your device or open it on your PC using a Java emulator.
(Note: Due to security policies, navigate to Archive.org and search for "Java ME Theme Utilities Collection Vol 3" to find the original file.)