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?
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!