fix(multiple): change fallbacks to use m3 (#29528)

Author: andrewseguin

src/material/core/_core.scss

@@ -7,6 +7,14 @@
 
 // Mixin that renders all of the core styles that are not theme-dependent.
 @mixin core() {
+  // TODO: Move ripple styles to be dynamically loaded instead of in core.
+  // This variable is used as a fallback for the ripple element's
+  // background color. However, if it isn't defined anywhere, then MSS
+  // complains in its verification stage.
+  html {
+    --mat-app-on-surface: initial;
+  }
+
   @include ripple.ripple();
   @include cdk.a11y-visually-hidden();
   @include cdk.overlay();

src/material/core/tokens/_token-utils.scss

@@ -113,11 +113,7 @@ $_system-fallbacks: m3-tokens.create-system-fallbacks();
     @return _create-var($sys-fallback, $fallback);
   }
 
-  // TODO(mat-app-theme): Return the system-level fallback.
-  // Changing this will affect clients that do not properly call theme mixins since the tokens
-  // will be undefined and now default to M3 system values, causing a number of screenshot failures.
-  // @return $sys-fallback;
-  @return $fallback;
+  @return $sys-fallback;
 }
 
 // Outputs a map of tokens under a specific prefix.