body {
    font-family: sans-serif;
}

h1 {
    text-align: center;
}

section, div, footer {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    row-gap: 0.5em;
    column-gap: 0.5em;
    margin: 1em auto;
    justify-content: center;
    text-align: center;
}

.grid {
    border-spacing: 0;
    border: 1px solid black;
    border-radius: 6px;
}
.grid td, tr {
    padding: 0;
}
.grid tr:first-child td:first-child {
    border-top-left-radius: 5px;
}
.grid tr:first-child td:first-child input {
    border-top-left-radius: 4px;
}
.grid tr:first-child td:last-child {
    border-top-right-radius: 5px;
}
.grid tr:first-child td:last-child input {
    border-top-right-radius: 4px;
}
.grid tr:last-child td:first-child {
    border-bottom-left-radius: 5px;
}
.grid tr:last-child td:first-child > input {
    border-bottom-left-radius: 4px;
}
.grid tr:last-child td:last-child {
    border-bottom-right-radius: 5px;
}
.grid tr:last-child td:last-child input {
    border-bottom-right-radius: 4px;
}
.grid tr:nth-child(3n+1) td {
    border-top: 1px solid black;
}
.grid tr:nth-child(3n+2) td {
    border-top: 1px solid grey;
    border-bottom: 1px solid grey;
}
.grid tr:nth-child(3n) td {
    border-bottom: 1px solid black;
}
.grid td:nth-child(3n+1) {
    border-left: 1px solid black;
}
.grid td:nth-child(3n+2) {
    border-left: 1px solid grey;
    border-right: 1px solid grey;
}
.grid td:nth-child(3n+3) {
    border-right: 1px solid black;
}

input[type=number] {
    width: 1.5em;
    height: 1.5em;
    font-size: 1.5em;
    border: 0;
    padding: 0;
    text-align: center;
    transition: background 0.5s;
    -moz-appearance: textfield;
}
input::-webkit-outer-spin-button,
input::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}
input::-webkit-calendar-picker-indicator {
    display: none;
}
input[type=number]:enabled {
    color: darkblue;
    background: white;
}
input[type=number]:disabled, button:enabled {
    color: white;
    background: #6666ff;
}
input[type=number].same-value:enabled {
    color: #009973 !important;
    background: #66ffd9 !important;
}
input[type=number].forbidden-value:enabled {
    background: #b3ffda !important;
}
input[type=number].same-value, button.same-value:enabled {
    color: white !important;
    background: #00e6ac !important;
}
input[type=number].forbidden-value:disabled {
    background: #6288ea !important;
}
input::placeholder {
    color: #888;
}

.highlight-buttons {
    column-gap: 0.3em;
}
button, input[type="color"] {
    border: 2px outset #6666ff;
    border-radius: 4px;
    font-size: 1em;
    margin: 0 0 1px 0;
}
button {
    padding: 0 8px 2px 8px;
}
button.same-value:enabled {
    border: 2px inset #00e6ac;
    padding: 2px 7px 0 9px;
    margin: 1px 0 0 0;
}
button:disabled {
    color: lightgrey;
    background: darkgrey;
    border: 2px outset darkgrey;
}
input[type="color"] {
    padding: 0;
}

a {
    text-decoration: none;
}