body {
    background-color: gray;
}

.blocks-container {
    display: block;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.blocks-row {
    display: flex;
}

.block {
    --block-size: 20px;
    border: 2px solid black;
    width: var(--block-size);
    height: var(--block-size);
}

.block.null {
    border-color: transparent;
}