mastodon/app/assets/stylesheets/components.scss

.status__content {
  a {
    color: #2b90d9;
    text-decoration: none;

    &:hover {
      text-decoration: underline;
    }

    &.mention {
      &:hover {
        text-decoration: none;

        span {
          text-decoration: underline;
        }
      }
    }
  }
}