Thanks (sorry that my question was a little heavy)
… I’m waiting then.
Today, I have found a way to have a specific _custom.scss for each multisites subdomain under the same antimatter theme (see structure in previous post above).
It was becoming necessary as the existing _custom.scss was becoming too big with 73ko stand alone!
nothing extraordinaray but as I am not a light of programmation,
… ☆ Very proud of that ♪
Long life to that extraordinary project: GRAV!!!