jupyter_notebook/static/tree/less/altuploadform.less

/* We need an invisible input field on top of the sentense*/
/* "Drag file onto the list ..." */

.alternate_upload {
    background-color:none;
    display: inline;

    &.form
    {
        padding: 0;
        margin:0;
    }

    input.fileinput
    {
        display: inline;
        opacity: 0;
        z-index: 2;
        width: 12ex;
        margin-right: -12ex;
    }

    .btn-upload {
        height: @btn_mini_height;
    }
}