If the themes are not applied, you can try several things.
- I made sure to have root permissions when open the module.
- Check that WhatsApp is closed, if it's not closed, you don't apply the change correctly (you can click the Apply Changes button located top right).
- Make sure the module is active in the module list Xposed.
- Whenever you install, remove, enable, or disable the module, you must restart your device.
- Load the theme Stock, and then load another.
- If all previous fails, delete the module and install a clean install by checking the above steps.