extract-custom-properties.js

const extractCustomProperties = (string = '') => Object.fromEntries(
  Array.from(
    string.matchAll(/--(?<property>[^:]+?)\s*:\s*(?<value>.*?)\s*[;}]/gus),
    ([match, property, value]) => [property, value]
  )
)

const css = `
  :root {
    --custom: property;
    --brand-color: #0c0;
  }
`

console.log(
  extractCustomProperties(css)
)

// {custom: "property", brand-color: "#0c0"}