fix(material/snack-bar): Emit tokens under mixin root selector (#27667)

Author: amysorto

src/material/snack-bar/_snack-bar-theme.scss

@@ -16,7 +16,7 @@
 @mixin color($config-or-theme) {
   $config: theming.get-color-config($config-or-theme);
 
-  .mat-mdc-snack-bar-container {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-snackbar-theme.theme(tokens-mdc-snack-bar.get-color-tokens($config));
     @include token-utils.create-token-values(
       tokens-mat-snack-bar.$prefix,
@@ -29,7 +29,7 @@
   $config: typography.private-typography-to-2018-config(
       theming.get-typography-config($config-or-theme));
 
-  .mat-mdc-snack-bar-container {
+  @include sass-utils.current-selector-or-root() {
     @include mdc-snackbar-theme.theme(tokens-mdc-snack-bar.get-typography-tokens($config));
   }
 }