const postcssCustomMedia = require('postcss-custom-media');
postcssCustomMedia.process(YOUR_CSS /*, processOptions, pluginOptions */);

const postcss = require('postcss');
const postcssCustomMedia = require('postcss-custom-media');
postcss([
  postcssCustomMedia(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

postcssCustomMedia({
  importFrom: 'path/to/file.css' // => @custom-selector --small-viewport (max-width: 30em);
});

postcssCustomMedia({
  importFrom: [
    'path/to/file.css',
    'and/then/this.js',
    'and/then/that.json',
    {
      customMedia: { '--small-viewport': '(max-width: 30em)' }
    },
    () => {
      const customMedia = { '--small-viewport': '(max-width: 30em)' };
      return { customMedia };
    }
  ]
});

postcssCustomMedia({
  exportTo: 'path/to/file.css' // @custom-media --small-viewport (max-width: 30em);
});

const cachedObject = { customMedia: {} };
postcssCustomMedia({
  exportTo: [
    'path/to/file.css',   // @custom-media --small-viewport (max-width: 30em);
    'and/then/this.js',   // module.exports = { customMedia: { '--small-viewport': '(max-width: 30em)' } }
    'and/then/this.mjs',  // export const customMedia = { '--small-viewport': '(max-width: 30em)' } }
    'and/then/that.json', // { "custom-media": { "--small-viewport": "(max-width: 30em)" } }
    cachedObject,
    customMedia => {
      customMedia    // { '--small-viewport': '(max-width: 30em)' }
    }
  ]
});