fix(material/card): Apply tokens at mixin root (#27557)

Applies card tokens at the theme mixin's root selector (or html if the
mixin is called with no selector). This makes it easier for users to
override tokens without worrying about specificity.

Author: mmalerba

src/material/card/_card-theme.scss

@@ -1,4 +1,5 @@
 @use 'sass:map';
+@use '../core/style/sass-utils';
 @use '../core/theming/theming';
 @use '../core/typography/typography';
 @use '../core/tokens/token-utils';
@@ -9,7 +10,7 @@
 @use '@material/card/outlined-card-theme' as mdc-outlined-card-theme;
 
 @mixin base($config-or-theme) {
-  .mat-mdc-card {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-elevated-card-theme.theme(tokens-mdc-elevated-card.get-unthemable-tokens());
     @include mdc-outlined-card-theme.theme(tokens-mdc-outlined-card.get-unthemable-tokens());
     @include token-utils.create-token-values(
@@ -32,7 +33,7 @@
   $mat-card-color-tokens: tokens-mat-card.get-color-tokens($config);
 
   // Add values for card tokens.
-  .mat-mdc-card {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-elevated-card-theme.theme($mdc-elevated-card-color-tokens);
     @include mdc-outlined-card-theme.theme($mdc-outlined-card-color-tokens);
     @include token-utils.create-token-values(tokens-mat-card.$prefix, $mat-card-color-tokens);
@@ -47,7 +48,7 @@
   $mat-card-typography-tokens: tokens-mat-card.get-typography-tokens($config);
 
   // Add values for card tokens.
-  .mat-mdc-card {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-elevated-card-theme.theme($mdc-elevated-card-typography-tokens);
     @include mdc-outlined-card-theme.theme($mdc-outlined-card-typography-tokens);
     @include token-utils.create-token-values(tokens-mat-card.$prefix, $mat-card-typography-tokens);
@@ -61,7 +62,7 @@
   $mat-card-density-tokens: tokens-mat-card.get-density-tokens($density-scale);
 
   // Add values for card tokens.
-  .mat-mdc-card {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-elevated-card-theme.theme($mdc-elevated-card-density-tokens);
     @include mdc-outlined-card-theme.theme($mdc-outlined-card-density-tokens);
     @include token-utils.create-token-values(tokens-mat-card.$prefix, $mat-card-density-tokens);