discourse/plugins/chat/assets/stylesheets/common/chat-modal-new-message.scss

.chat-modal-new-message {
  & + .modal-backdrop {
    opacity: 1;
    background: transparent;
  }

  .modal-body {
    padding: 0;
  }

  .modal-header {
    display: none;
  }

  .modal-inner-container {
    width: var(--modal-max-width);
    box-shadow: var(--shadow-dropdown);
    overflow: hidden;
  }

  .mobile-device & {
    .modal-inner-container {
      border-radius: 0;
      margin: 0 auto auto auto;
      box-shadow: var(--shadow-modal);
    }
  }

  .not-mobile-device & {
    .modal-inner-container {
      margin: 10px auto auto auto;
    }
  }
}