Remove setting the dark-mode explicitly (#100)

The dark mode feature is now enabled by default.

[Source](https://tailwindcss.com/docs/upgrade-guide#remove-dark-mode-configuration)

Author: dixpac

lib/install/tailwind.config.js

@@ -1,12 +1,11 @@
 const defaultTheme = require('tailwindcss/defaultTheme')
 
 module.exports = {
-  darkMode: 'media',
   content: [
     './app/helpers/**/*.rb',
     './app/javascript/**/*.js',
     './app/views/**/*'
-  ],  
+  ],
   theme: {
     extend: {
       fontFamily: {