**GetSimple CMS** --> http://get-simple.info/

They claim: "The Simplest Content Management System. Ever."

And I would have to agree.  After downloading the code it was up and running in a matter of minutes. Very impressed with the ease of installer, and the ease of customization.  

On other flat file CMS I used the Atom Editor to do custom tweaks of the templates. No need for any additional tools here. If you know just a little about HTML and CSS the admin interface allows you to very easily customize the look and feel. 

https://www.opensourcecms.com/getsimple-cms/

With out a doubt the most simple and lightweight CMS is WonderCMS.  If you are looking for just a few page website, to just have an online presence, or perhaps something just a tad bit more than a tombstone page to park your domain, WonderCMS is well worth giving a try.

https://www.opensourcecms.com/

I really like GetSimple CMS as a CMS. What I found amusing is that GetSimple CMS uses DokuWiki for their instruction manual