First I have created an active_admin css file as app/assets/stylesheets/active_admin.css.scss
/* active_admin.css.scss */
@import \"active_admin
This is the link that addresses this issue. I am reposting an existing answer that I found very helpful because it also addressed the js issues. Again Not My Answer.
https://github.com/activeadmin/activeadmin/issues/3819#issuecomment-351377822
arif362 commented on Dec 13, 2017
the same problem I faced and I have solved this following below:
Firstly you have to create a directory named assets in the vendor directory,
Then in the assets directory of vendor folder, you have to create two directories (i) stylesheets and the second one is (ii) javascripts.
After that, you have to move the active_admin.scss file from assets/stylesheets to vendor/assets/stylesheets and the active_admin.js.coffee file from assets/javascripts to vendor/assets/javascripts,
And finally, restart your rails server then reload your apps. Hope this will work for you.