If you test the start orb by switching to another theme and it doesn't change with the theme and you have already tried to revert to the default start orb in the app for manually changing the start orb, follow these steps:

1. Search "cmd" in start menu, right click Command Prompt and "Run as administrator"
2. Type sfc/scannow and press enter.
3. Do not close the command prompt and allow the scan to complete.
4. Reboot and change themes, if the start orb is not changing still with custom themes, reinstall Theme Resource Changer.