[joy-ui] `--joy-spacing` CSS var no longer injected

[Onurfesci]

Steps to reproduce

Link to live example (Any of the Joy UI Codesandbox):
https://codesandbox.io/s/pt8svk?file=/src/Demo.tsx

Steps:

1. Update to latest version of @mui/joy
2. --joy-spacing CSS var is no longer injected into styles

Current behavior

--joy-spacing CSS var is not injected into styles

Expected behavior

--joy-spacing CSS var is injected into styles

Context

I used var(--joy-spacing) in quite a few of my custom styles rather than theme -> theme.spacing(x) to minimize JS and it worked nicely.

Your environment

npx @mui/envinfo

  System:
    OS: macOS 14.6.1
  Binaries:
    Node: 21.6.0 - /usr/local/bin/node
    npm: 10.2.4 - /usr/local/bin/npm
    pnpm: 9.6.0 - /usr/local/bin/pnpm
  Browsers:
    Chrome: 128.0.6613.84
    Edge: 128.0.2739.42
    Safari: 17.6

I used Chrome.

Search keywords: joy, spacing, theme

[siriwatknp]

Thanks for reporting the issue! Joy UI development is temporarily on hold as the maintainers focus on the next two major releases of Material UI