fix(material-experimental/mdc-progress-bar): use color.adjust

Author: andrewseguin

src/material-experimental/mdc-progress-bar/_progress-bar-theme.scss

@@ -9,7 +9,7 @@
   // writing, their buffer color is hardcoded to #e6e6e6 which both doesn't account for theming
   // and doesn't match the Material design spec. For now we approximate the buffer background by
   // applying an opacity to the color of the bar.
-  $buffer-color: rgba(mdc-theme-prop-value($color), 0.25);
+  $buffer-color: color.adjust(mdc-theme-prop-value($color), $alpha: -0.75);
   @include mdc-linear-progress-bar-color($color, $query: mdc-helpers.$mat-theme-styles-query);
   @include mdc-linear-progress-buffer-color($buffer-color,
     $query: mdc-helpers.$mat-theme-styles-query);