WordPress - open-source CMS
WordPress has been around since 2011 and has dominated the website market for years.
The structure of WordPress is spot on. Themes control how the website looks. Plugins control what the website can do. Your content (data) remains as separated as possible from both of these concerns. This means you can switch themes and plugins without losing your website. (But always back up first!)
The WordPress™ framework itself is open-source and therefore free. There are hundreds of free themes. There are thousands of free plugins. You can do just about anything without spending money on either.
WordPress uses PHP and MySQL under the hood, but don't worry. You don't need to learn either. Plugins and themes do most of the work for you. If you need a custom plug-in Paul can make that happen too.