FlatPress is a lightweight, easy-to-set-up blogging engine. Plain and simple, just PHP. No database needed!
It's best suitable for personal blogs or small business sites. FlatPress is a single user system (see Tech FAQ) and does not offer an editorial workflow. But would you need that?
FlatPress works great out of the box, but you may also extend it to your needs with plugins and themes.
Important to know: FlatPress is not “WordPress on flat files”. WordPress plugins usually don't work in FlatPress. Anyway, sometimes you are just a few lines away from making them work :) In fact, many of the bundled filters on the text are from WP.
Themes have just nothing to do with WP, not at all.
Why should one even use a self-hosted blogging system in times of huge easy-to-use platforms like Facebook, Instagram and Twitter? Why hosting the comments for yourself when you could do so with Disqus?
A personal blog in reasonable size is just not the kind of application that would need a database beneath it. Our project site flatpress.org contains more than 220 blog entries since 2006 and runs as quickly as a virgin install. No flat-file performance problems whatsoever!