* {
    border-style: solid;
    border-color: red;
}

.container ul {
    list-style: none;
}

.Welcome .container {
    display: none;
}


/* Larger than mobile */

@media (min-width: 401px) {
    /*CSS styling for desktop mode here*/
}

@media (min-width: 1240px) {
    .Welcome .container {
        display: block;
    }
    .Welcome .container img {
        position: relative;
        width: 100%;
        /* for IE 6 */
    }
    h1 {
        position: absolute;
        top: 100px;
        left: 420px;
    }
}

* {
    border-style: solid;
    border-color: red;
}



.container {
  border-color: blue;
  width: 100%;
}

.container ul {
    list-style: none;
}

.Welcome .container {
    display: none;
}



/* Larger than mobile */

@media (min-width: 401px) {
    /*CSS styling for desktop mode here*/
}

@media (min-width: 1240px) {
    .Welcome .container {
        display: block;
    }
    .Welcome .container img {
        position: relative;
        width: 100%;
        /* for IE 6 */
    }
    h1 {
        position: absolute;
        top: 100px;
        left: 420px;
    }
}

Make cotainer width 100% on mobile screen
Make cotainer width 100% on mobile screen
