Following the instructions for the bootstrap-sass Gem, we start with an app/assets/stylesheets/application.css.scss that looks like:
@import "bootstrap-sprockets";
@import "bootstrap";
Say we want to use the Yeti theme, download the _bootswatch.scss and _variables.scss files from the Bootswatch site. Place the files in a vendor/assets/stylesheets/bootswatch/yeti/ directory in your Rails project.
Finally add the new @import statements in your app/assets/stylesheets/application.css.scss file, leaving it looking like:
@import "bootswatch/yeti/_variables";
@import "bootstrap-sprockets";
@import "bootstrap";
@import "bootswatch/yeti/_bootswatch";
The order of the imports is important. I have a comment block above those imports that reads:
// order is important here
// 1. theme variable overrides
// 2. theme variables
// 3. any core bootstrap variable overrides
// 4. bootstrap-sockets and bootstrap
// 5. theme styles
// 6. application styles