First I have created an active_admin css file as app/assets/stylesheets/active_admin.css.scss
app/assets/stylesheets/active_admin.css.scss
/* active_admin.css.scss */ @import \"active_admin
Remove this line from your app/assets/stylehseets/application.css.sass
*= require_tree ../../../vendor/assets/stylesheets/
Move active_admin.css.scss to vendor/assets/stylesheets/