style.css

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Radio Canada', sans-serif;
}

input, button {
    outline: none;
    border: none;
}

.mainBox {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    width: 100%;
    height: 100vh;
    transition: 1s;
}

.inputRGBText {
    cursor: default;
    user-select: text;
    width: 545px;
    text-transform: lowercase;
    text-align: center;
    height: 100px;
    font-size: 52px;
    letter-spacing: 0;
    transition: 1s;
    background: #f2f2f2;
}

.randomButton {
    cursor: pointer;
    position: absolute;
    text-align: center;
    text-transform: capitalize;
    user-select: none;
    top: 60%;
    width: 350px;
    border-radius: 50px;
    height: 85px;
    font-size: 45px;
    font-weight: bold;
    transition: 0.3s;
    background: #f2f2f2;
}

.copyImg {
    cursor: pointer;
    position: absolute;
    left: 70vw;
    width: 105px;
    height: 105px;
    background: white;
    border-radius: 15px;
    border: white 10px solid;
}

@media screen and (min-width: 300px) and (max-width: 800px) {
    .mainBox {
        width: auto;
    }

    .inputRGBText {
        transform: translateY(-50%);
        width: 315px;
        font-size: 33px;
        border: white 10px solid;
    }

    .randomButton {
        transform: translateY(45%);
        margin: -10px 0px;
        width: 350px;
    }

    .copyImg {
        display: block;
        width: 60px;
        height: 60px;
    }
}