As we know Active Admin (https://activeadmin.info) is a popular Ruby on Rails plugin to generate admin interface. It has many benefit over currently existing other similar plugin like Rails Admin. Active Admin gives the facility to customize pages, layouts.
Here is simple demo of Active Admin with default theme http://demo.activeadmin.info/admin.
Beside this default theme, you can find many different Active Admin themes. My favorite theme is active_admin_flat_skin theme. It gives layout with different color pellet.
Installation and using process for this gem is similar to other Ruby gems which you can also find in Github documentation.
gem 'active_admin_flat_skin', '~>0.1.2'
Here are few screenshot of Active Admin panel using this theme.
Also you can check all available themes from this GitHub repository (https://github.com/paladini/activeadmin-themes) .