Here's a simple setup guide for integrating Active Admin 4, Rails 8, and Tailwind CSS

_html_head.html.erb

<!-- views/active_admin/_html_head.html.erb -->
<%= stylesheet_link_tag :app %>
<meta name="viewport" content="width=device-width,initial-scale=1">
<%= csrf_meta_tags %>
<%= csp_meta_tag %>
<% # On page load or when changing themes, best to add inline in `head` to avoid FOUC %>
<%= javascript_tag nonce: true do %>
  if (localStorage.theme === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
  document.documentElement.classList.add('dark')
  } else {
  document.documentElement.classList.remove('dark')
  }
<% end %>
<%= javascript_importmap_tags "active_admin", importmap: ActiveAdmin.importmap %>

tailwind.config.js

// config/tailwind.config.js

const defaultTheme = require('tailwindcss/defaultTheme')
const execSync = require('child_process').execSync;
const activeAdminPath = execSync('bundle show activeadmin', { encoding: 'utf-8' }).trim();

module.exports = {
  content: [
    './public/*.html',
    './app/helpers/**/*.rb',
    './app/javascript/**/*.js',
    './app/views/**/*.{erb,haml,html,slim}',
    `${activeAdminPath}/vendor/javascript/flowbite.js`,
    `${activeAdminPath}/plugin.js`,
    `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
    './app/admin/**/*.{arb,erb,html,rb}',
    './app/views/active_admin/**/*.{arb,erb,html,rb}',
    './app/views/admin/**/*.{arb,erb,html,rb}',
    './app/views/layouts/active_admin*.{erb,html}',
    './app/javascript/**/*.js'
  ],
  theme: {
    extend: {
      fontFamily: {
        sans: ['Inter var', ...defaultTheme.fontFamily.sans],
      },
    },
  },
  darkMode: "selector",
  plugins: [
    require('@tailwindcss/forms'),
    require('@tailwindcss/typography'),
    require('@tailwindcss/container-queries'),
    require(`${activeAdminPath}/plugin`) 
  ]
}

zsh

# zsh

rails new app_name -c tailwind
cd app_name
bundle add activeadmin --version "~> 4.0.0.beta14"
rails g active_admin:install
rm tailwind-active_admin.config.js