CSS 사용자 이름 색상이 관리자/모더레이터에게만 적용되는 것인가요?

If you want to target custom groups, you need to install this theme component, as stated by Moin: CSS Classes for Current User's Groups. It will insert the current user’s groups into the body HTML.

Then, you can target with .group-NAME:

.group-NAME {
  .names {
     span a::after {
       content:"★";
       margin-left: 2px;
     }
  }
}