feat: Add an option to use darkMode only in the widget (#7151)

tailwind.config.js

@@ -1,6 +1,7 @@
 const { colors } = require('tailwindcss/defaultTheme');
 
 module.exports = {
+  dark: 'class',
   purge: [
     './app/javascript/widget/**/*.vue',
     './app/javascript/portal/**/*.vue',
@@ -11,6 +12,9 @@ module.exports = {
   future: {
     removeDeprecatedGapUtilities: true,
   },
+  experimental: {
+    darkModeVariant: true,
+  },
   theme: {
     colors: {
       white: colors.white,