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).
Most of the broken content is at the bottom of individual post pages and is mainly Jetpack content – comments, related posts, etc. The one exception is the author bio, which I can’t blame on Jetpack. I had similar issues with the Atomic Blocks theme and they resolved this with some CSS updates to the theme – hopefully Twenty Nineteen can do the same (yes, I have reported the problems).
Have a look and consider trying it yourself.