Their Github page says:
'The Free version, for everyday use, is the basic version at the Google Play Store. It is compiled and signed by the developers, and sent to the Play Store to be cataloged by Google.
The Libre version is the version provided to have no proprietary code, and is not linked to any closed services. The app is automatically signed and compiled from the GitHub repository by F-Droid. It is possible to unlock the themes with a procedure.'