fix: enable in shadow DOM

because:
- the `:root` selector does not work in the context of a
shadow DOM

this commit:
- extends the selector with the `:root` selector, so the
CSS variables are known inside a shadow DOM

src/generic/_generic.ms-custom-properties.scss

@@ -6,7 +6,8 @@
  * Generate the custom properties necessary to calculate the different ratios
  * at runtime in CSS.
  */
-:root {
+:root,
+:host {
     @each $scale in map-get($FLUIDMS-CONFIG, scales) {
         --ms#{$scale}-ratio:       #{fluidms-pow(map-get($FLUIDMS-CONFIG, ratio), $scale)};
         @if $FLUIDMS-GLOBAL-BASELINE {