* {
  box-sizing: border-box;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
  background: linear-gradient(135deg, #74b9ff, #a29bfe);
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}

.screen {
  background: white;
  padding: 30px;
  border-radius: 16px;
  box-shadow: 0 8px 25px rgba(0, 0, 0, 0.15);
  width: 360px;
  text-align: center;
}

.hidden {
  display: none;
}

h1 {
  color: #2d3436;
  margin-bottom: 5px;
}

.subtitle {
  color: #636e72;
  font-size: 0.9rem;
  margin-bottom: 20px;
}

.form-group {
  margin-bottom: 15px;
  text-align: left;
}

.form-group label {
  display: block;
  font-size: 0.85rem;
  font-weight: bold;
  color: #2d3436;
  margin-bottom: 5px;
}

.form-group input, .form-group select {
  width: 100%;
  padding: 10px;
  border-radius: 8px;
  border: 1px solid #dfe6e9;
  font-size: 0.95rem;
  outline: none;
  background-color: #f1f2f6;
}

.btn-main {
  width: 100%;
  padding: 12px;
  font-size: 1rem;
  font-weight: bold;
  color: white;
  background-color: #00b894;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  transition: 0.2s;
  margin-top: 10px;
}

.btn-main:hover {
  background-color: #00a884;
}

.btn-secondary {
  padding: 6px 12px;
  font-size: 0.85rem;
  font-weight: bold;
  color: #636e72;
  background-color: #dfe6e9;
  border: none;
  border-radius: 6px;
  cursor: pointer;
}

.btn-secondary:hover {
  background-color: #b2bec3;
}

/* Phần Chủ quyền */
.copyright-box {
  margin-top: 25px;
  padding-top: 15px;
  border-top: 1px solid #dfe6e9;
  font-size: 0.8rem;
  color: #636e72;
  line-height: 1.4;
}

/* Giao diện Game Screen */
.game-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 20px;
}

.game-header h2 {
  font-size: 1.1rem;
  color: #2d3436;
  margin: 0;
}

.board {
  display: grid;
  grid-template-columns: repeat(3, 95px);
  grid-template-rows: repeat(3, 95px);
  gap: 8px;
  margin: 0 auto 20px auto;
  justify-content: center;
}

.cell {
  background-color: #f1f2f6;
  border-radius: 8px;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 2.5rem;
  font-weight: bold;
  cursor: pointer;
  transition: background 0.2s;
}

.cell:hover {
  background-color: #dfe6e9;
}

.cell.x { color: #e74c3c; }
.cell.o { color: #3498db; }