LH 2015 is a theme for community organisations, originally forked from Twenty Fifteen and now standalone. It keeps Twenty Fifteen’s clean two-column layout: the site’s logo, title, tagline and menus sit in a fixed left column beside the content, and collapse behind a Menu button on small screens.
It is a hybrid theme. The header and footer are template parts built entirely from core blocks (Site Logo, Site Title, Site Tagline, Navigation and a Button), so each site edits them in the Site Editor and fills its own menus there. Most views still use the theme’s PHP templates; wherever a post type has a block template, registered by a plugin, shipped in the theme or saved in the Site Editor, that is used instead, with the same header and footer.