position fixed en index y centrado

<style>
.fixed-box {
    position: fixed;
    right: 1em;
    bottom: 1em;
    width: 40%;
    height: 40%;
}
.center-box {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
    width: 100%;
    height: 100%;
}
</style>