chore(website): fix text color for animating app bar

Author: mlaursen

packages/documentation/src/components/Demos/AppBar/AnimatingAppBar.module.scss

@@ -29,7 +29,7 @@
   $height: (3.5 * 16) + 96; // app-bar-height (3.5rem) + 96 for custom offset
 
   @include rmd-button-theme-update-var(color, $rmd-white-base);
-  @include rmd-theme(color, $rmd-white-base);
+  @include rmd-theme(color, text-primary-on-dark);
 
   background-image: url(#{'https://picsum.photos/#{$width}/#{$height}?image=811'});
   height: auto;