Wow there are dev OSes on the public internet domain?

No. Or it would be a great mistake by TI. Or a leak by some testers which would definitely cause them troubles....
What you can find tho, are screenshots and photos of devices running prototypes/dev OSes.

And I wonder what would TI stand to gain by releasing this theme editor? They don't really release appearance features such as these.
Shame it won't work to a great effect on non-CXes tho...

I think they did that in order to test different theme variations directly by changing them on-calc, without having to recompile the OS each time they wanted to try new stuff.
I wouldn't bet on a release of that tool anytime.

I wouldn't bet on a release of that tool anytime.

Awww, that would have been the very best!!! A release of the theme editor, able to change the TI-nspire theme TI has "forced" on us. But it looks not to be.

Custom backgrounds will be nice (instead of that plain black background that somewhat resembles older iPhones)

I still sense Juju will try to make a custom theme with flying ponies (like on his android phone) if such tool ever arrives

Anime theme .  Or Omnimaga/ TI calculator theme