We are developing a web application using Ruby on rails.
In order to create an administrator screen, I created an administrator screen by referring to the following site.

Reference site
https://qiita.com/enomotodev/items/5f6d9348207124a41bf9

I succeeded in creating the administrator screen, but it has nothing to do with the administrator screen
When I jumped to the localhost: 3000/home/index page etc., the color was different from before.
At first glance, I feel that the activeadmin style has been applied.

There was also a problem that the a tag was underlined even though the css style was applied, such as not underlining the a tag.

How can I solve this problem?
Someone please professor

application.scss
/ *
 * This is a manifest file that'll be compiled into application.css, which will include all the files
 * listed below.
 *
 * Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
 * vendor/assets/stylesheets directory can be referenced here using a relative path.
 *
 * You're free to add application-wide styles to this file and they'll appear at the bottom of the
 * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
 * files in this directory. Styles in this file should be added after the last require_ * statement.
 * It is generally better to create a new file per style scope.
 *
 * = require_tree.
 * = require_self
 * /
 @import "bootstrap";
 a {
   text-decoration: none;
 }