style.css

* {
    margin: 0;
    padding: 0;
    user-select: none;
    box-sizing: border-box;
}

body {
    height: 300vh;
    background: url(img/bg.jpg);
    background-position: center !important;
    background-size: cover !important;
    background-attachment: fixed !important;
}

section {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100vh;
}

section .side {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

section .side#side1 {
    background: white;
    clip-path: inset(0 50% 0 0);
}

section .side#side2 {
    background: red;
    clip-path: inset(0 0 0 50%);
}

.cross {
    position: absolute;
    top: 25px;
    left: 25px;
    background: gray;
    width: 250px;
    height: 250px;
    clip-path: polygon(10% 25%, 35% 25%, 35% 0%, 65% 0%, 65% 25%, 90% 25%, 90% 50%, 65% 50%, 65% 100%, 35% 100%, 35% 50%, 10% 50%);
}

@media screen and (min-width: 300px) and (max-width: 800px) {
    .cross {
        top: 10px;
        left: 10px;
        width: 150px;
        height: 150px;
    }

    body {
        background: url(img/bg2.jpg);
    }
}