Replace :host elements with a prefix of your choosing
const prefix = require('postcss-prefix')
const postcss = require('postcss')
const css = `
:host { color: blue }
.hello { color: black }
`
const newCss = postcss([ prefix('#hello-world') ])
.process(css)
.toString()
console.log(newCss)
// => #hello-world { color: blue }
// => .hello { color: black }
`