.contenedor_flex_box
{
     display: flex;
    flex-wrap: wrap;
    flex-direction: row;
    justify-content: start;
    align-items: start;
    align-content: start
}
    .item
    {

        border: 2px solid white;
        flex-grow: 1 !important;
        flex-shrink: 1 !important;
        flex-basis: 300px !important;
        height: auto;
        margin: 20px !important;
    }

    body{

        background-image: url("../images/corcho.jpg");
        background-repeat: repeat;
    }