.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 1px;
    background-color: #334155; /* border color */
}

.cell {
    aspect-ratio: 1;
    text-align: center;
    font-size: 1.25rem;
    font-weight: bold;
    color: #1e293b;
    background-color: white;
    border: none;
    outline: none;
    cursor: pointer;
}

.dark .cell {
    background-color: #1e293b;
    color: #f1f5f9;
}

.cell:focus {
    background-color: #e0e7ff;
}
.dark .cell:focus {
    background-color: #312e81;
}

/* Thicker borders for 3x3 blocks */
.cell:nth-child(3n) {
    border-right: 2px solid #334155;
    margin-right: -1px; /* overlap gap */
}
.cell:nth-child(9n) {
    border-right: none;
    margin-right: 0;
}
/* Rows */
.sudoku-grid input {
    /* Using CSS Grid gap handling in JS helps, but pure CSS nth-child logic for row borders is tricky with flat inputs. 
       Easier to rely on the grid gap. 
       But we want strictly 3x3 visual blocks. 
       Let's stick to simple gap=1px slate-500. 
       We will add border-bottom to row 3, 6 via JS classes or nth-child.
    */
}

.cell.border-r-thick {
    border-right: 2px solid #0f172a;
}
.cell.border-b-thick {
    border-bottom: 2px solid #0f172a;
}

.cell.fixed {
    color: #0f172a;
    background-color: #f1f5f9;
}
.dark .cell.fixed {
    color: #94a3b8;
    background-color: #0f172a;
}

.cell.error {
    color: #e11d48;
    background-color: #ffe4e6;
}

.numpad-btn {
    padding: 1rem;
    background: #f1f5f9;
    border-radius: 0.5rem;
    font-weight: bold;
    font-size: 1.25rem;
}
.dark .numpad-btn {
    background: #1e293b;
    color: white;
}