fix(material-experimental/theming): disabled radio button incorrect label color (#28415)

Fixes that disabled radio buttons didn't have the correct color of the label when disabled. Also switches some existing `rgba` calls to use the `safe-color-change` function.

Author: crisbeto

src/material-experimental/theming/_custom-tokens.scss

@@ -526,8 +526,8 @@
 /// @param {Boolean} $exclude-hardcoded Whether to exclude hardcoded token values
 /// @return {Map} A set of custom tokens for the full variant of mat-pseudo-checkbox
 @function full-pseudo-checkbox($systems, $exclude-hardcoded) {
-  $disabled-color-base: map.get($systems, 'md-sys-color', 'on-surface');
-  $disabled-color: if($disabled-color-base == null, null, rgba($disabled-color-base, 0.38));
+  $disabled-color: mat.private-safe-color-change(
+    map.get($systems, md-sys-color, on-surface), $alpha: 0.38);
 
   @return (
     selected-icon-color: map.get($systems, 'md-sys-color', 'primary'),
@@ -544,12 +544,10 @@
 /// @param {Boolean} $exclude-hardcoded Whether to exclude hardcoded token values
 /// @return {Map} A set of custom tokens for the minimal variant of mat-pseudo-checkbox
 @function minimal-pseudo-checkbox($systems, $exclude-hardcoded) {
-  $disabled-color-base: map.get($systems, 'md-sys-color', 'on-surface');
-  $disabled-color: if($disabled-color-base == null, null, rgba($disabled-color-base, 0.38));
-
   @return (
     selected-checkmark-color: map.get($systems, 'md-sys-color', 'primary'),
-    disabled-selected-checkmark-color: $disabled-color,
+    disabled-selected-checkmark-color: mat.private-safe-color-change(
+      map.get($systems, md-sys-color, on-surface), $alpha: 0.38),
   );
 }
 
@@ -561,7 +559,8 @@
   @return (
     ripple-color: map.get($systems, md-sys-color, on-surface),
     checked-ripple-color: map.get($systems, md-sys-color, primary),
-    disabled-label-color: map.get($systems, md-sys-color, on-surface),
+    disabled-label-color: mat.private-safe-color-change(
+      map.get($systems, md-sys-color, on-surface), $alpha: 0.38),
   );
 }
 
@@ -628,8 +627,8 @@
 /// @return {Map} A set of custom tokens for the mat-slider
 @function slider($systems, $exclude-hardcoded) {
   $ripple-color: map.get($systems, md-sys-color, primary);
-  $hover-ripple-color: if($ripple-color == null, null, #{rgba($ripple-color, 0.05)});
-  $focus-ripple-color: if($ripple-color == null, null, #{rgba($ripple-color, 0.2)});
+  $hover-ripple-color: mat.private-safe-color-change($ripple-color, $alpha: 0.05);
+  $focus-ripple-color: mat.private-safe-color-change($ripple-color, $alpha: 0.2);
 
   @return (
     ripple-color: $ripple-color,