If you're on GNOME, you can just install that extension. If you use Budgie, you should file a feature request at https://github.com/solus-project/budgie-desktop. For other desktops, their respective bug trackers.
You could also write a script that changes your GTK theme and write some systemd timers that trigger at certain times. That'd basically do the same thing, excluding the manual graphical toggle.
