Bootstrap Theme : Bootswatch

Bootswatch http://bootswatch.com/ ; free theme

To leverage that we have a gem that is designed to do just that.

Add the following in your Gemfile

 gem 'bootswatch-rails'

Do a bundle install

 bundle install 

Browse to the following folder, app/stylesheets/application.css.scss

 @import 'bootswatch/simplex/variables'; 
 @import "bootswatch/simplex/bootswatch" 

Now test to see if this works. if you are looking to change the theme, just replace simplex with the name of the theme from Bootswatch.