Planned this long time ago, needed much time to finish it (due to business work) - but I like the result: My small, long-living blog is now a modern, mobile-first with self-made CSS-Grid Drupal 8 website :-) I've ignored the Microsoft fixes, so you need an updated, modern browser on the desktop to see the sidebar not below but aside the main content.
Because I'm using Cloudflare on this domain I've didn't do everything which was in my head. The design is mostly like the Drupal 7 Theme Esteem. On Google Pagespeed Insights I get now 79 (needs Work) instead of 61 (Poor) for mobile, for desktops 93 (Good) points instead of 71 (needs Work).
As I'll change small things here and there I've also made two screenshots (mobile and desktop) of the "right now" version - and when you slide to the right you'll see two screenshots of the Drupal 7 version.