/*plus petit que 1024*/
@media only screen
and (max-width : 1024px){
    #col_principal{
        width:auto;
        float:none;
    }

    #sidebar{
        float:none;
        width:100%;
    }
}

/*entre 768 et 1024*/
@media only screen
and (min-width: 769px)
and (max-width : 1024px){
    
}

/* plus petit que 768*/
@media only screen
and (max-width : 768px) {
    
}

@media only screen
and (max-width : 320px) {
   
}