WordPress is the most popular content management system (CMS) in the world. It is free and open source software released in 2003 by Matt Mullenweg.
It can be customized to fit any needs, from a one-man operation to a large enterprise.
There are many free WordPress themes, but the most customizable is probably Divi. It is available as a free plugin and as a free theme for WordPress.
It is highly customizable and can be used for a wide range of sites. Divi also has a large community of developers who can help with any issues you may have.