You’ll need to add.
In post #42, the code under ‘stylesheet’ will need to go in your child theme reboot.php (or reboot-child.php, whatever it’s called)
And the code under ‘overlay’ will need to be added to each overlay for which you want to be able to change the colours.
In fact, that might be a bit of a snag. Give it a try, and if it works OK test some of the overlays that don’t have the change, it might throw a few warnings..