<div class="block"></div>

body {
    min-height: 100vh;
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
}

.block {
    --radius: 5px;
    --size: 20px;

    width: 160px;
    height: 160px;
    border-radius: 5px;
    background-color: royalblue;
    background-image: radial-gradient(circle at center, #eeeeee 0, #eeeeee var(--radius), transparent calc(var(--radius) + 0.785px));
    background-size: var(--size) var(--size);
}