:root {
  --bg-color: #020617;
  --panel-bg: rgba(15, 23, 42, 0.7);
  --text-main: #f8fafc;
  --text-muted: #94a3b8;
  --border-color: rgba(255, 255, 255, 0.3);
  --border-thick: rgba(255, 255, 255, 0.8);
  --body-bg: radial-gradient(circle at center, #0f172a 0%, #020617 100%);
  --select-bg: rgba(0,0,0,0.3);
  --btn-bg: rgba(255,255,255,0.08);
  --btn-border: rgba(255,255,255,0.15);
  --btn-hover: rgba(255,255,255,0.15);
  --modal-overlay: rgba(0,0,0,0.6);
  
  /* Additional variables from shudu */
  --cell-bg: rgba(0, 0, 0, 0.4);
  --cell-hover: rgba(0, 0, 0, 0.2);
  --cell-highlighted: rgba(56, 189, 248, 0.15);
  --cell-selected: rgba(56, 189, 248, 0.4);
  --cell-same: rgba(56, 189, 248, 0.25);
  --color-given: #f8fafc;
  --color-input: #38bdf8;
  --color-error: #ef4444;
  --color-success: #22c55e;
  --selected-shadow: inset 0 0 0 2px rgba(255,255,255,0.4);
}

:root.light-mode {
  --bg-color: #f8fafc;
  --panel-bg: rgba(255, 255, 255, 0.85);
  --text-main: #0f172a;
  --text-muted: #475569;
  --border-color: rgba(0, 0, 0, 0.15);
  --border-thick: rgba(0, 0, 0, 0.4);
  --body-bg: radial-gradient(circle at center, #f1f5f9 0%, #cbd5e1 100%);
  --select-bg: rgba(255,255,255,0.6);
  --btn-bg: rgba(0,0,0,0.05);
  --btn-border: rgba(0,0,0,0.1);
  --btn-hover: rgba(0,0,0,0.1);
  --modal-overlay: rgba(255,255,255,0.5);

  --cell-bg: rgba(255, 255, 255, 0.6);
  --cell-hover: rgba(0, 0, 0, 0.05);
  --cell-highlighted: rgba(56, 189, 248, 0.2);
  --cell-selected: rgba(56, 189, 248, 0.4);
  --cell-same: rgba(56, 189, 248, 0.3);
  --color-given: #0f172a;
  --color-input: #0284c7;
  --selected-shadow: inset 0 0 0 2px rgba(0,0,0,0.4);
}

body {
  margin: 0;
  padding: 2rem 0;
  box-sizing: border-box;
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background: var(--body-bg);
  color: var(--text-main);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  overflow-y: auto;
  user-select: none;
}

.game-container {
  margin: auto;
  background: var(--panel-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border: 1px solid rgba(255, 255, 255, 0.1);
  border-radius: 24px;
  padding: 2rem;
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
  max-width: 100vw;
  box-sizing: border-box;
  width: 90%;
  max-width: 500px;
}

h1 {
  margin: 0;
  font-size: 1.8rem;
  font-weight: 700;
  letter-spacing: 2px;
  background: linear-gradient(to right, #38bdf8, #818cf8);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  text-align: center;
}

.btn {
  background: var(--btn-bg);
  border: 1px solid var(--btn-border);
  color: var(--text-main);
  border-radius: 12px;
  padding: 1rem 0;
  font-size: 1.3rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: inherit;
}

.btn-block {
  width: 100%;
  max-width: 300px;
}

.btn:hover {
  background: var(--btn-hover);
  transform: translateY(-2px);
}

.btn:active {
  transform: translateY(1px);
}

.btn.primary {
  background: linear-gradient(135deg, #38bdf8, #818cf8);
  border: none;
  box-shadow: 0 4px 15px rgba(56, 189, 248, 0.3);
  color: #fff;
}

.btn.primary:hover {
  box-shadow: 0 6px 20px rgba(56, 189, 248, 0.5);
}

@media (max-width: 550px) {
  .game-container { 
    padding: 1rem 0.5rem; 
    border-radius: 0; 
    width: 100%; 
    height: 100%; 
    justify-content: flex-start; 
    border: none; 
    box-shadow: none; 
    background: transparent; 
    backdrop-filter: none; 
  }
  h1 { font-size: 1.5rem; }
  body { justify-content: flex-start; }
  .btn { padding: 0.8rem 0; font-size: 1.2rem; }
}