Ok, new ControlX option let you switch your calculator between release and development modes :
In development mode, you can install and run development OSes, like the CX 3.0.0.1045 OS with the integrated theme editor.
This is done by patching the 2048-bits RSA keys used for validating TI-Nspire.cer in the Boot2.
It should be possible to install and run CM OSes soon, as the problem is also related to different RSA keys.