Update: I’ve now reverted back to my previous theme. Twenty Nineteen had too many issues present for me to continue using it (it was the first, very draft release, after all). Everything I saw has been reported though and I’m looking forward to using it a later time.
Just 3 hours ago (at the time of writing), the new theme for WordPress 5.0 was announced – Twenty Nineteen. And I’m running the very early version on this site.
Designed specifically for Gutenberg, it looks great but, yeah, some if it has broken the site (you don’t need to tell me).