is a customizable theme for your web application
Activo is a theme written in SASS. It's meant to be used with the HAML or ERB templates generated by the Web-app-theme plugin. Web-app-theme is a rails plugin that you can use to generate a complete web application layout.
But there's more! It offers you the css you need to use with another cool gem, Formtastic! Formtastic makes it easier to create forms in your Rails applications.
Activo also has the styles for the Attrtastic gem, which allows you to easily create index/show pages.
Want more? Jon Wood made helpers for menus, breadcrumbs, and controls!