*,
*::before,
*::after {
    box-sizing: border-box;
}

body {
    margin: 0;
    min-height: 100vh;
    font-family: system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
    background: #f5f5f5;
    color: #212121;
}

header {
    background: #3f51b5;
    color: #fff;
    padding: 1rem 1.25rem;
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
}

header h1 {
    margin: 0;
    font-size: 1.35rem;
    font-weight: 500;
    text-align: center;
}

main {
    max-width: 28rem;
    margin: 0 auto;
    padding: 1.5rem 1rem 2rem;
    text-align: center;
}

.examples {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: center;
    gap: 0.5rem 0.75rem;
    margin-bottom: 1rem;
    font-size: 0.9rem;
}

.examples label {
    color: #616161;
    font-weight: 500;
}

.examples select {
    font-family: inherit;
    font-size: 0.9rem;
    padding: 0.4rem 0.6rem;
    border: 1px solid #bdbdbd;
    border-radius: 4px;
    background: #fff;
    color: #212121;
    cursor: pointer;
    max-width: 100%;
}

.examples select:focus {
    outline: none;
    border-color: #3f51b5;
    box-shadow: 0 0 0 2px rgba(63, 81, 181, 0.25);
}

/* 9×9 CSS grid — thick lines mark 3×3 boxes */
.grid {
    display: grid;
    grid-template-columns: repeat(9, 2.5rem);
    grid-template-rows: repeat(9, 2.5rem);
    gap: 0;
    width: fit-content;
    margin: 0 auto 1.25rem;
    background: #212121;
    border: 3px solid #212121;
    border-radius: 2px;
}

.cell {
    width: 2.5rem;
    height: 2.5rem;
    margin: 0;
    padding: 0;
    border: 1px solid #bdbdbd;
    border-radius: 0;
    text-align: center;
    font-size: 1.25rem;
    font-weight: 500;
    font-variant-numeric: tabular-nums;
    background: #fff;
    color: #212121;
    outline: none;
    -moz-appearance: textfield;
}

.cell::-webkit-outer-spin-button,
.cell::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}

.cell:focus {
    background: #e8eaf6;
    z-index: 1;
    box-shadow: inset 0 0 0 2px #3f51b5;
}

.cell.given {
    font-weight: 700;
    color: #1a237e;
}

.cell.solved {
    color: #2e7d32;
    background: #e8f5e9;
}

.cell.just-placed {
    background: #c8e6c9;
    box-shadow: inset 0 0 0 2px #43a047;
    animation: pulse-placed 0.45s ease-out;
}

@keyframes pulse-placed {
    from {
        background: #a5d6a7;
    }
    to {
        background: #c8e6c9;
    }
}

.cell.border-left {
    border-left: 2px solid #212121;
}

.cell.border-top {
    border-top: 2px solid #212121;
}

.cell.border-right {
    border-right: 2px solid #212121;
}

.cell.border-bottom {
    border-bottom: 2px solid #212121;
}

.cell.border-left-outer {
    border-left: 2px solid #212121;
}

.cell.border-top-outer {
    border-top: 2px solid #212121;
}

.actions {
    display: flex;
    flex-wrap: wrap;
    gap: 0.75rem;
    justify-content: center;
    margin-bottom: 0.75rem;
}

.btn {
    cursor: pointer;
    padding: 0.6rem 1.4rem;
    font-size: 0.95rem;
    font-weight: 500;
    font-family: inherit;
    border: none;
    border-radius: 4px;
    background: transparent;
    color: #3f51b5;
    transition: background 0.15s, box-shadow 0.15s;
}

.btn:hover:not(:disabled) {
    background: rgba(63, 81, 181, 0.08);
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.btn-primary {
    background: #3f51b5;
    color: #fff;
    box-shadow: 0 1px 3px rgba(0, 0, 0, 0.2);
}

.btn-primary:hover:not(:disabled) {
    background: #303f9f;
}

.status {
    min-height: 1.4em;
    margin: 0.5rem 0 1rem;
    font-size: 0.95rem;
    color: #616161;
}

.status.error {
    color: #c62828;
}

.footer {
    margin: 0;
    font-size: 0.9rem;
}

.footer a {
    color: #3f51b5;
    text-decoration: none;
}

.footer a:hover {
    text-decoration: underline;
}

@media (max-width: 400px) {
    .grid {
        grid-template-columns: repeat(9, 2.1rem);
        grid-template-rows: repeat(9, 2.1rem);
    }

    .cell {
        width: 2.1rem;
        height: 2.1rem;
        font-size: 1.1rem;
    }
}