chore(cna): fix theme extend for tailwind v4 (#76583)
## Why?
We are not extending the tailwind theme for `--color-background` and `--color-foreground`.
(Before)
https://github.com/user-attachments/assets/d336dab3-a00f-46cb-9587-055a6c5272fa
(After)
https://github.com/user-attachments/assets/123e829d-aa7e-4b57-aa21-261fc05350f8
Follow-up fix for https://github.com/vercel/next.js/pull/75407.
