[v4] @theme --background-size-name not working.

[demonguyj]

What version of Tailwind CSS are you using?
4.0.0 beta.6

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

Next.js 15.1.0

What version of Node.js are you using?

For example: v20.18.0

What browser are you using?

Chrome

What operating system are you using?

macOS

Reproduction URL

A Tailwind Play link or public GitHub repo that includes a minimal reproduction of the bug. Please do not link to your actual project, what we need instead is a minimal reproduction in a fresh project without any unnecessary code. This means it doesn't matter if your real project is private/confidential, since we want a link to a separate, isolated reproduction anyways.

A reproduction is required when filing an issue — any issue opened without a reproduction will be closed and you'll be asked to create a new issue that includes a reproduction. We're a small team and we can't keep up with the volume of issues we receive if we need to reproduce each issue from scratch ourselves.

Describe your issue

I wrote
@theme {
--background-size-full: 100%;
}

It is expected to:

.bg-full {
background-size:100%;
}

but it is not created.

[wongjn]

The background size utilities are registered as static utilities:

tailwindcss/packages/tailwindcss/src/utilities.ts

Lines 2271 to 2273 in 4277857

	staticUtility('bg-auto', [['background-size', 'auto']])
	staticUtility('bg-cover', [['background-size', 'cover']])
	staticUtility('bg-contain', [['background-size', 'contain']])

Which means you can't configure new ones in @theme. However, you could add your own bg-full via a CSS rule:

.bg-full {
  background-size:100%;
}

Or as a utility:

@utility bg-full {
  background-size:100%;
}

[demonguyj]

I created

@Utility bg-full {
background-size:100%;
}

And I tried apply to before:bg-full.
It isn't worked...

But .bg-full is working.