[development] Update on 5.0 core themes

[Khalid B]

Well, we already have more than one theme in core, so why not
in 5.0? If it is because they are not ready, then, yes, it is a valid
point.

To the authors/maintainers/designers of the themes that don't make
it to core, I salute you just as much for your work, and please add
your theme to the contrib repository regardless.

Perhaps Trae's version of Zen should go into contrib too as a Zen
variant.