Txs a lot @Ariba for your kind follow-up.
I have investigated further on my side.
And I do confirm that there is some kinf of issue when Helix Ultimate compiles SCSS (when using for example rgba and lighten).
See the following example (which I have added on my live website mentioned above so that you can see for yourself) :
$joomshaper: #69aa41;
$joomshaper-rgba: rgba($joomshaper, 0.5);
$joomshaper-lighten: lighten($joomshaper,20%);
.joomshaper {background: $joomshaper};
.joomshaper-rgba {background: $joomshaper-rgba};
.joomshaper-lighten {background: $joomshaper-lighten};
Test it with https://www.sassmeister.com/, it compiles nicely into
.joomshaper {background: #69aa41;}
.joomshaper-rgba {background: rgba(105, 170, 65, 0.5);}
.joomshaper-lighten {background: #9fcf82;}
But in https://www.healthybelgium.be/templates/shaper_helixultimate/css/template.css?2105275327 it compiles into
.joomshaper {background: #69aa41;}
.joomshaper-rgba {background: rgba(#69aa41, 0.5);}
.joomshaper-lighten { background: lighten(#69aa41, 20%);}
So I guess there is a problem somewhere in the SCSS/SAAS compiler coming along with Helix Ultimate :)