#modal_avisos{
    width: 600px;
}

  @media only screen and (max-width : 900px) 
  {
    #modal_avisos
    {
    width: 100%;
    }
  .modal-content
    {
        background-color: #ffffff00;
    }
    .modal-dialog
    {
        background-color: none;
        border: none !important;
    }
  }