I really like the way Medium posts look. I think the Medium team has done a great job spending time on making sure a post looks its best no matter what the content it contains.
When I was setting up this blog, I looked around for any theme that was just barebones, the current theme I'm using is called Integer by ThemePatio. It was one of the few that I found that doesn't only look Perfect™ when looking at the preview. Most themes will look very good when you're looking at a preview of them and they're not designed for regular blogging, they're designed for cookie-cutter blog posts. I think this is unfair to the people looking at the themes since it means people will be oversold on a theme and when they install it, it'll look like it's missing something.
I am starting to work on a custom theme that I'm going to be making available. It's called Free Press, you can see it on github here: usmanity/free-press. 🗞
The goal is to keep the theme very minimal, maybe take a few design hints from Medium but overall stay the course of simplicity.