discourse/plugins/chat/assets/stylesheets/common/chat-thread-participants.scss

.chat-thread-participants {
  &__other-count {
    font-size: var(--font-down-2);
    color: var(--primary-high);
    white-space: nowrap;
  }

  &__avatar-group {
    display: flex;
    align-items: center;
    justify-content: flex-end;

    .chat-user-avatar__container {
      padding: 0;
    }

    .chat-user-avatar {
      width: auto !important;

      .avatar {
        padding: 0;
      }
    }
  }
}

@container (max-width: 400px) {
  .chat-thread-participants {
    &__avatar-group {
      flex-direction: row;
      justify-content: flex-start;

      .chat-user-avatar {
        &:not(:last-child) {
          margin-right: -10px;
        }
        .avatar {
          border: 1px solid var(--primary-very-low);
        }
      }
    }
  }
}