Sudoku/style.css

body {
    font-family: sans-serif;
}

h1 {
    text-align: center;
}

div {
    display: flex;
    column-gap: 0.5em;
    margin: 1em auto;
    justify-content: center;
    text-align: center;
}

table {
    border-spacing: 0;
    border: 1px solid black;
    border-radius: 6px;
}

td, tr {
    padding: 0;
}

tr:first-child td:first-child {
    border-top-left-radius: 5px;
}

tr:first-child td:first-child input {
    border-top-left-radius: 4px;
}

tr:first-child td:last-child {
    border-top-right-radius: 5px;
}

tr:first-child td:last-child input {
    border-top-right-radius: 4px;
}

tr:last-child td:first-child {
    border-bottom-left-radius: 5px;
}

tr:last-child td:first-child > input {
    border-bottom-left-radius: 4px;
}

tr:last-child td:last-child {
    border-bottom-right-radius: 5px;
}

tr:last-child td:last-child input {
    border-bottom-right-radius: 4px;
}

tr:nth-child(3n+1) td {
    border-top: 1px solid black;
}

tr:nth-child(3n+2) td {
    border-top: 1px solid grey;
    border-bottom: 1px solid grey;
}

tr:nth-child(3n) td {
    border-bottom: 1px solid black;
}

td:nth-child(3n+1) {
    border-left: 1px solid black;
}

td:nth-child(3n+2) {
    border-left: 1px solid grey;
    border-right: 1px solid grey;
}

td:nth-child(3n+3) {
    border-right: 1px solid black;
}

input {
    width: 1.6em;
    height: 1.6em;
    font-size: 1.5em;
    border: 0;
    padding: 0;
    text-align: center;
    transition: 0.3s;
}

input:enabled {
    color: darkblue;
    background: white;
}

input:disabled {
    color: white;
    background: #6666ff;
}

.same-value:enabled {
    color: #009973;
    background: #66ffd9;
}

.forbidden-value:enabled {
    background: #b3ffda;
}

.same-value:disabled {
    color: white;
    background: #00e6ac;
}

.forbidden-value:disabled {
    background: #6288ea;
}

input::placeholder {
    color: #888;
}

.buttons {
    column-gap: 0.2em;
    margin: 0;
}

input[type="color"] {
    font-size: 1em;
}

a {
    text-decoration: none;
}