Open edXposed Manager, install the latest version of KMod FWA with edXposed Manager open, when installed, deactivate KMod FWA from the module list, activate it again and reboot the normal device (not software). Get into updates and update libraries.
If you have Android 8 or above, please enable permissive SELinux from KMod FWA setting.
If none of this works, check the FAQs in case I have missed something, because I can't think of anything else, maybe just that you update to the latest version of WhatsApp. Although if the themes are working for you, I don't understand what the problem is, what doesn't work exactly?
Thank you, I forgot this option, I think it was disabled by the update.
Maybe you can add it to FAQs.
I also can confirm the bug of the connection status toast mentioned by death_jax
Lastly, I suggest you to add a search engine for costumization options, if you think it's a good idea and if you want!