Now I´m totally stuck by extending/changing the Antimatter skeleton and couldn´t find a solution here in the forum (which is really great!). I´m running the modular page plus the blog in one installation. With “onpage_menu: true” and adding following in ‘site.yaml’:
menu: - text: Blog class: "" url: blog
Blog is placed correctly in the modular menu, by hovering the menu item the link is displayed correctly as well but by clicking the link nothing happens. By adding an additional ‘blog_navigation.html.twig’ I prevented that the Blog entry from ‘site.yaml’ will show up in the blog site twice. All looks good but the ‘Blog’ link within the modular page menu doesn´t work.
With the hint from @flaviocopes in the post “GRAV One-Page Antimatter Theme” dated Nov. 10, 2015 I´ve been trying to search for a CSS z-index problem. Without any success. I don´t get any error messages and I´m really lost. Any idea how to get the menu link to ‘02.blog page’ working?