Issue with SheetRegistry.toString({format: false}) minifyer - part of the css is not minified

__Expected behavior:__
Minify all the server-side CSS.

__Describe the bug:__
Font faces and CSS variables are not getting minified In Next.js app.

__Reproduction:__
Create CSS file with CSS variables and font faces, and get the registry string with `Registry.toString({format: false})`

__Versions (please complete the following information):__
- **react-jss**: 10.8.0
- Browser [e.g. chrome, safari]: chrome
- OS [e.g. Windows, macOS]: macOS
Feel free to add any additional versions which you may think are relevant to the bug.

__Managing expectations:__

Maintainers will not be fixing the problem you have unless they have it too, if you want it to get fixed:

1. Submit a PR with a failing test
2. Discuss a solution
3. Implement it

You can also do the first step only and wait for someone else to work on a fix. Anything is much better than nothing.