Hey! :) Themes are already implemented, everything will be customizable with tilesets and setting desired hex colors! Regarding pre-orders, it grants complete access to both the engine and all its updates, no limitations. But it says pre-order because the early access is not ready yet, sorry! Check the roadmap post in the community if you want a cleare overview of the situation.
I'm very very glad you like this project, if you think it's going to be useful, please, share it with friends :)