Fix Tailwind 'purge' -> 'content'

See https://tailwindcss.com/docs/upgrade-guide#configure-content-sources.

web/tailwind.config.js

@@ -2,8 +2,10 @@ const defaultTheme = require('tailwindcss/defaultTheme')
 
 module.exports = {
   mode: 'jit',
-  purge: ['./pages/**/*.{js,ts,jsx,tsx}', './components/**/*.{js,ts,jsx,tsx}'],
-  darkMode: 'media', // checks CSS @media(prefers-color-scheme)
+  content: [
+    './pages/**/*.{js,ts,jsx,tsx}',
+    './components/**/*.{js,ts,jsx,tsx}',
+  ],
   theme: {
     fontFamily: Object.assign(
       { ...defaultTheme.fontFamily },