Run these from your browser's JS console to get a report of everything JS can read from CSSOM

list-colors-used-on-site.js

import(`https://unpkg.com/cssomtools`).then(({process, property}) => {
  const found = new Set

  process(
    [
      property('color', true),
      property('background-color', true)
    ],
    rule => {
      if (rule.style.color) {
        found.add(rule.style.color)
      }
      if (rule.style.backgroundColor) {
        found.add(rule.style.backgroundColor)
      }
    }
  )

  console.group('Colors used on site')
  console.log(found)
  console.groupEnd()
})

list-fonts-used-on-site.js

import(`https://unpkg.com/cssomtools`).then(({process, property}) => {
  const found = new Set()

  process(
    [
      property('font', true),
      property('font-family', true)
    ],
    rule => {
      if (rule.style.fontFamily) {
        found.add(rule.style.fontFamily)
      }
      if (rule.style.font) {
        found.add(rule.style.fontFamily)
      }
    }
  )

  console.group('Fonts used on site')
  console.log(found)
  console.groupEnd()
})

list-media-queries-used-on-site.js

import('https://unpkg.com/cssomtools').then(({process, query}) => {
  const found = new Set

  process(
    query(),
    rule => {
      if (
        rule.media
        && rule.media.mediaText
      ) {
        found.add(rule.media.mediaText)
      }
    }
  )

  console.group('Media query breakpoints used on site')
  console.log(found)
  console.groupEnd()
})