Manually building `public/css/style.css` using latest `sass` release fails because of `_mixins.css` not using `.scss` extension

[Missingmew]

I am currently setting up nitter on a system using Oracle Linux 9. It lacks the libsass package so nimble scss fails because it doesnt find the library (obviously). Grabbing the latest standalone release of sass from https://github.com/sass/dart-sass as recommended by https://sass-lang.com/install/ and replacing nimble.scss with sass src/sass/index.scss public/css/style.css --load-path=src/sass/include fails as follows:

Error: This at-rule isn't allowed in plain CSS.
  ╷
3 │ @mixin panel($width, $max-width) {
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ╵
  _mixins.css 3:1                @import
  src/sass/tweet/_base.scss 2:9  @import
  src/sass/index.scss 3:9        root stylesheet

I noticed that _mixins.css was not using the .scss extension. After a mv src/sass/include/_mixins.css src/sass/include/_mixins.scss the build works just fine.

The sass webpage suggests that libsass is deprecated and was lagging behind the other implementations as well. I assume that libsass working just fine with the "wrong" extension is one example of this.