[v4] Using PostCSS @theme doesn't work in a Next.js project

[Facug03]

What version of Tailwind CSS are you using?

v4.0.0-alpha.6

What build tool (or framework if it abstracts the build tool) are you using?

tailwindcss/postcss: 4.0.0-alpha.6, Next.js 14.1.3

What version of Node.js are you using?

v18.17.1

What browser are you using?

Firefox

What operating system are you using?

Windows

Reproduction URL

https://github.com/Facug03/tailwind-v4-next

It's a repo that i created only to try the new version

Describe your issue

I installed the most recent version of Next.js, followed by the installation of the latest version of Tailwind using PostCSS, and it worked. However, when attempting to utilize the new '@theme', it threw an error:

CSS:

[adamwathan]

Fixed in #13162, thanks for reporting! Publishing a new release in a sec.