LH 2015 is a child theme of Twenty Fifteen for community organisations. It keeps Twenty Fifteen’s clean two-column layout, with the site branding, menus and widgets in a fixed left column, and adds the things a club or community site needs: a custom logo in the sidebar, extra widget areas, a footer menu, lighter vanilla JavaScript in place of the parent’s jQuery scripts, and support for full-width blocks.
It is also a hybrid theme. Every page uses its classic PHP templates, except where a post type has a block template of its own: one registered by a plugin, a templates/single-{post_type}.html file in the theme, or one saved from the template editor. Those pages get the same sidebar column and footer through header and footer template parts, so a plugin can ship a block template for its post type and it will look at home.
Twenty Fifteen must be installed, since LH 2015 uses it as its parent theme.