*{
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body{
    background: #dcdcdc;
}

.a{
    background: #39ff14;
    border: solid 2px black;
    min-height: 100px;

    display: flex;
    justify-content: center;
    align-items: center;
}

.B{
    background: #19b5fe;
    border: solid 2px black;
    min-height: 100px;

    display: flex;
    justify-content: center;
    align-items: center;
}
 @media (min-width: 10px) and (max-width: 450px) {

            .a{
                height: 200px;
                background-color: blueviolet;
            }

        }

        @media (min-width: 450px) and (max-width: 1024px) {

            .a{
                height: 200px;
                background-color: rgb(43, 220, 226);
            }

        }