See article: https://medium.com/consciousapps/exporting-css-from-webflow-into-your-app-a7a08e3c7731

medium-article-selenium.js

(async () => {
	const webdriver = require('selenium-webdriver')
	const chrome = require('selenium-webdriver/chrome')
	const chromedriver = require('chromedriver')

	// Run Selenium with Chrome on Node
	chrome.setDefaultService(new chrome.ServiceBuilder(chromedriver.path).build())

	let driver = new webdriver.Builder()
		.withCapabilities(webdriver.Capabilities.chrome())
		// but make it headless, meaning we won’t see a browser window open
		.setChromeOptions(new chrome.Options().headless())
		.build()
	try {
		// Head on over to the Webflow site
		await driver.get('https://__WEBFLOW_SITE__.webflow.io')
		// Find and open the CSS file...
		const cssFile = await driver.findElement(webdriver.By.tagName('link')).getAttribute('href')
		await driver.get(cssFile)
		// ...get its contents...
		const allCss = await driver.findElement(webdriver.By.tagName('pre')).getAttribute('innerText')
		// ...and, finally, just get the part that’s your own custom css.
		const customCss = allCss.split(
			'========================================================================== */'
		)[3]
		// Overwrite your custom css file on your hard drive with your new css
		const fs = require('fs')
		fs.writeFileSync(
			// Path to your CSS file on your hard drive; to get the path of your css file, drag it from Finder into Terminal (on Mac)
			'/Users/__PATH_TO_YOUR_CSS_FILE__/src/assets/css/index.css',
			customCss
		)
	} finally {
		// and be sure to quit Selenium
		await driver.quit()
	}
})()