bootswatchフォルダ作成
app/assets/stylesheets/bootswatch
そのフォルダにbootswatchの
variables.lessとbootswatch.lessを投入
bootstrap_and_overrides.css.lessに2行追加
@import "./bootswatch/variables";
@import "./bootswatch/bootswatch";
application.css変更
*= require_tree .
↓
*= require "bootstrap_and_overrides"
参考
http://piko.hateblo.jp/entry/2012/09/22/180712
