What version of Tailwind CSS are you using?

v4.0.3

What version of Node.js are you using?

v23.7.0

What operating system are you using?

Arch Linux

Reproduction URL

https://github.com/fiws/tailwind-issue-f1

Describe your issue

After running npx @tailwindcss/upgrade@next, my "dark" theme config in the old tailwind.config.js was transformed into @custom-variant dark (@media not print { .dark & }); this does not work and appears to be invalid syntax. See below for details:

before

https://github.com/fiws/tailwind-issue-f1/blob/676604340b5ad3ef2384ea86ab79d755a82dfb40/tailwind.config.js#L7

// tailwind.config.js
darkMode: ["variant", "@media not print { .dark & }"],

after

https://github.com/fiws/tailwind-issue-f1/blob/main/src/index.css#L3

@custom-variant dark (@media not print { .dark & });

I also have no idea what to correct syntax is..