I've add rules in pre.scss and in post.scss
In post.scss :
In pre.scss, I've defined one new color :
$blue: #c1ddfb ;
-> With that code, my theme gets new button color or menu color and my H1 text is orange.
But when I put all the other bootstrap variables in pre.scss (from https://github.com/moodle/moodle/blob/master/theme/boost/scss/bootstrap/_variables.scss ), all my settings are gone (h1 is black, my blue color is back to the old one).
I can't understand why, because there is only one $blue in pre.scss, and I thought that my post.scss should override all the scss as it come after.