Enable tailwind dark: and light: mode prefix

Hello, since the 1.8rc0 was out, DaisyUI was noted to have the benefit of light and dark mode.

From the elixir subreddit, it seem a few people (two) realize the dark and light class prefix is not enabled by default and require an additional line to the css file:

@custom-variant dark (&:where([data-theme=dark], [data-theme=dark] *));

I think it would be a nice quality of life to have the line in the app.css file by default.

Thank you

Would it be better to open a pull request?