Use text changing function

ui.js

window.onmessage = async event => {
  const message = event.data.pluginMessage
  if (message.type === 'selectionChange') {
    // this 👇
    const contrast = document.getElementById('contrast')
    contrast.textContent = message.contrast
    // becomes this 👇 
    changeText('contrast', message.contrast)
  }
}