As mentioned in official document you need to import theme variables

page css file path:
src/components/pages/login/page.css

@reference "../../../app/globals.css";

h3 {
  @apply bg-new-theme-color;
}

global css file path:
src/app/globals.css

@import "tailwindcss";

@theme {
--color-new-theme-color: #fbfbaa;
}

I hope its helpful