:root {
    /* Default: Light Mode (Premium Professional Look) */
    --bg-primary: #f8fafc;
    --bg-secondary: #ffffff;
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --accent-primary: #3b82f6;
    --accent-hover: #2563eb;
    --board-border: #94a3b8;
    --cell-bg: #ffffff;
    --cell-hover: #f1f5f9;
    --cell-selected: #bae6fd;
    --cell-same-number: #e2e8f0;
    --cell-pink: #FFBFEC;
    --text-given: #000000;
    --text-user: #2563eb;
    --success: #22c55e;
    --panel-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
}

/* Dark Mode Overrides */
body.dark-mode {
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --text-primary: #f8fafc;
    --text-secondary: #94a3b8;
    --board-border: #334155;
    --cell-bg: #0f172a;
    --cell-hover: rgba(59, 130, 246, 0.1);
    --cell-selected: rgba(59, 130, 246, 0.3);
    --cell-same-number: rgba(59, 130, 246, 0.1);
    --cell-pink: rgba(236, 72, 153, 0.15);
    --text-given: #ffffff;
    --text-user: #60a5fa;
}

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
    font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
    background-color: var(--bg-primary);
    color: var(--text-primary);
    height: 100vh;
    overflow: hidden; /* Dwingt alles binnen het frame */
    transition: background 0.3s ease;
}

.app-container {
    height: 100vh;
    display: flex;
    flex-direction: column;
    padding: 10px 25px;
    gap: 12px;
}

/* --- HEADER: THE PERFECT LINE --- */
.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 12px 35px;
    background: var(--bg-secondary);
    border-radius: 16px;
    border: 1px solid var(--board-border);
    box-shadow: var(--panel-shadow);
}

.logo h1 {
    font-size: 2.6rem;
    font-weight: 900;
    background: linear-gradient(135deg, #3b82f6, #8b5cf6);
    background-clip: text;
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    display: flex;
    align-items: center;
    gap: 10px;
}

.badge {
    background: linear-gradient(135deg, #c084fc, #ec4899);
    padding: 4px 12px;
    border-radius: 8px;
    font-size: 1.1rem;
    font-weight: bold;
    -webkit-text-fill-color: white;
}

/* Header Center Controls */
.header-center {
    display: flex;
    align-items: center;
    gap: 35px;
}

.control-group {
    display: flex;
    flex-direction: row;
    align-items: center;
    gap: 15px;
}

.label-text {
    font-size: 1.2rem;
    font-weight: 700;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.header-select, .header-btn {
    height: 52px;
    padding: 0 20px;
    border-radius: 12px;
    border: 2px solid var(--board-border);
    background: var(--bg-primary);
    color: var(--text-primary);
    font-size: 1.2rem;
    font-weight: 600;
    cursor: pointer;
    outline: none;
    transition: border-color 0.2s;
}

.header-select:hover { border-color: var(--accent-primary); }
.header-select { min-width: 200px; }

.btn-primary.header-btn {
    background: var(--accent-primary);
    color: white;
    border: none;
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.3);
}

.btn-primary.header-btn:hover {
    background: var(--accent-hover);
    transform: translateY(-1px);
}

/* Header Right: Settings Clusters */
.header-right {
    display: flex;
    gap: 15px;
}

.settings-cluster {
    display: flex;
    background: var(--bg-primary);
    padding: 4px;
    border-radius: 12px;
    border: 1px solid var(--board-border);
}

.icon-btn {
    background: transparent;
    border: none;
    font-size: 1.8rem;
    width: 60px;
    height: 52px;
    border-radius: 10px;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    opacity: 0.3;
    transition: 0.2s;
}

.icon-btn.active {
    opacity: 1;
    background: var(--bg-secondary);
    box-shadow: 0 2px 8px rgba(0,0,0,0.1);
}

.icon-btn:hover { opacity: 1; }

/* --- MAIN LAYOUT --- */
.main-content { flex: 1; display: flex; flex-direction: column; min-height: 0; position: relative; }
.game-layout { flex: 1; display: flex; gap: 25px; min-height: 0; align-items: stretch; }

/* The Board */
.board-container {
    flex: 1;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 10px;
}

.board {
    display: grid;
    background: var(--board-border);
    border: 4px solid var(--board-border);
    position: relative;
    gap: 1px;
    box-shadow: 0 20px 50px rgba(0,0,0,0.1);
}

/* --- SUDOKU X DIAGONAL LINES --- */

/* De basis voor beide lijnen */
.board.variant-x::before,
.board.variant-x::after {
    content: '';
    position: absolute;
    background-color: var(--accent-primary);
    opacity: 0.15; /* Subtiele lijn zodat cijfers leesbaar blijven */
    height: 6px;   /* Iets dikker voor goede zichtbaarheid op groot bord */
    width: 141.42%; /* De wortel van 2 (Pythagoras) om de hoek te halen */
    top: 0;
    left: 0;
    transform-origin: 0 0;
    z-index: 5;    /* Bovenop de cellen */
    pointer-events: none; /* Zorgt dat je door de lijn heen kunt klikken */
    display: none; /* Standaard uit */
}

/* Lijn van linksboven naar rechtsonder \ */
.board.variant-x::before {
    display: block;
    transform: rotate(45deg);
}

/* Lijn van linksonder naar rechtsboven / */
.board.variant-x::after {
    display: block;
    top: 100%;
    transform: rotate(-45deg);
}

/* Extra highlight voor de cellen die op de diagonaal liggen (optioneel maar mooi) */
.board.variant-x .cell-diagonal {
    background-color: rgba(59, 130, 246, 0.05);
}

.cell {
    background: var(--cell-bg);
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: background 0.1s;
    color: var(--text-user);
    font-weight: 400;
}

.cell.given { color: var(--text-given); font-weight: 600; }
.cell.selected { background: var(--cell-selected) !important; }
.cell.same-number { background: var(--cell-same-number); }
.cell.pink-mode { background: var(--cell-pink) !important; }

/* Sharp Subgrids (The 9 Boxes) */
.thick-right { border-right: 4px solid var(--board-border) !important; }
.thick-bottom { border-bottom: 4px solid var(--board-border) !important; }

/* --- SIDEBAR --- */
.sidebar { width: 350px; display: flex; flex-direction: column; gap: 15px; }

.panel {
    background: var(--bg-secondary);
    padding: 15px 20px;
    border-radius: 20px;
    border: 1px solid var(--board-border);
    box-shadow: var(--panel-shadow);
}

h3 {
    margin-bottom: 18px;
    font-size: 1.1rem;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 1px;
    font-weight: 800;
}

/* Numpad: Maximum Visibility */
.numpad {
    display: grid;
    gap: 10px;
    margin-bottom: 15px;
}

.num-btn {
    background: var(--bg-primary);
    border: 2px solid var(--board-border);
    color: var(--text-primary);
    font-size: 1.6rem; /* Iets kleiner font */
    padding: 10px; /* Minder padding (was 20) */
    aspect-ratio: 1 / 1; /* Dwingt vierkante knoppen */
    border-radius: 10px;
    cursor: pointer;
    font-weight: 700;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.2s ease;
}

.num-btn:hover {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
    color: white;
    transform: translateY(-2px);
}

/* Buttons */
.btn {
    padding: 14px 20px;
    border-radius: 12px;
    cursor: pointer;
    font-weight: 800;
    border: none;
    font-size: 1.1rem;
    transition: 0.2s;
}

.btn-secondary { background: var(--bg-primary); border: 2px solid var(--board-border); color: var(--text-primary); }
.btn-outline { background: transparent; border: 2px solid var(--board-border); color: var(--text-primary); }
.btn-outline:hover { background: var(--bg-primary); }
.w-full { width: 100%; }

/* Note Grid */
.notes-grid { display: grid; width: 100%; height: 100%; }
.note { display: flex; align-items: center; justify-content: center; color: var(--text-secondary); line-height: 1; }

/* Toggles */
.toggle-group { display: flex; align-items: center; gap: 15px; margin-bottom: 12px; font-size: 1.1rem; font-weight: 600; }
.toggle-switch { position: relative; width: 48px; height: 26px; }
.toggle-switch input { opacity: 0; width: 0; height: 0; }
.slider { position: absolute; cursor: pointer; inset: 0; background-color: #cbd5e1; transition: .3s; border-radius: 26px; }
.slider:before { position: absolute; content: ""; height: 20px; width: 20px; left: 3px; bottom: 3px; background-color: white; transition: .3s; border-radius: 50%; }
input:checked + .slider { background-color: var(--accent-primary); }
input:checked + .slider:before { transform: translateX(22px); }

/* Feedback & UI */
.game-status {
    position: absolute;
    top: 100px;
    left: 50%;
    transform: translateX(-50%);
    z-index: 1000;
    background: var(--success);
    color: white;
    padding: 20px 60px;
    border-radius: 50px;
    font-size: 1.8rem;
    font-weight: 900;
    box-shadow: 0 10px 40px rgba(34, 197, 94, 0.4);
}

.loading-overlay { position: fixed; inset: 0; background: var(--bg-primary); display: flex; align-items: center; justify-content: center; z-index: 9999; }
.spinner { width: 60px; height: 60px; border: 6px solid var(--board-border); border-top-color: var(--accent-primary); border-radius: 50%; animation: spin 0.8s linear infinite; }
@keyframes spin { to { transform: rotate(360deg); } }

.hidden { display: none; }