Mosaic is a new WordPress page builder from Nextend Web, the team behind Smart Slider 3. It installs as a plugin but you need to pick one of the Mosaic themes. Mosaic launched with lots of advanced features including a style system that uses CSS variables and classes, components, a drag and drop builder, element option inputs for CSS layout and styling, and an extensive library of predesigned assets. There is extensive support for dynamic data and ACF.
Master pages have a header and footer which you can enable as the default site wide, or you can have more than one Master page for different parts of your site. Mosaic has four themes, each of which has its own style variables and library. You can export your own theme, which will include the master pages, style system settings, templates, and components.