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
What worked for me (Rails 5) was adding the following line on my app/assets/stylesheets/application.css after the *= require_self line:
*= require_self
*= stub "active_admin"