:root {
  --electric-blue: #00BFFF; 
  --dark-blue: #004d99;  
  --white: #f0f0f0;     
  --main-bg: #1a1a1a;   
  --cell-bg: #1a1a1a;   
  --initial-cell-bg: #2c2c2c; 
  --internal-border-color: #555; 
}

.section-title {
    color: var(--electric-blue);
    font-size: 2em;
    margin-bottom: 15px;
}

#daily-sudoku-section {
    display: flex;
    flex-direction: column;
    align-items: center;
    width: 100%;
    max-width: 600px;
    margin: 20px auto;
    padding: 15px;
    border: 2px solid var(--dark-blue);
    border-radius: 8px;
    background-color: var(--initial-cell-bg);
}

#daily-sudoku-title {
    color: var(--electric-blue);
    font-size: 2em;
    margin-bottom: 10px;
    text-shadow: 0 0 5px rgba(0, 191, 255, 0.5);
}

#daily-sudoku-info {
    color: var(--white);
    margin-bottom: 15px;
    font-size: 1.1em;
}


#game-container {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 25px;
  margin: 0 auto 50px auto; 
  max-width: 600px;
  padding: 20px;
}

#game-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  width: 100%;
  color: var(--white);
  font-size: 1.2em;
}

#difficulty-selector {
  padding: 8px;
  font-size: 1em;
  border-radius: 5px;
  background-color: var(--dark-blue);
  color: var(--white);
  border: 1px solid var(--electric-blue);
  cursor: pointer;
}

#sudoku-grid {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(9, 1fr);
  width: 100%;
  max-width: 450px;
  aspect-ratio: 1 / 1;
  border: 3px solid var(--electric-blue); 
  background-color: var(--initial-cell-bg); 
}

.sudoku-cell {
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 1.5em;
  border: 1px solid var(--internal-border-color); 
  background-color: var(--cell-bg);
  color: var(--white);
  transition: background-color 0.2s ease, transform 0.1s ease;
  cursor: pointer;
}

.sudoku-cell:hover {
  background-color: #333;
}

.sudoku-cell:nth-child(3n):not(:nth-child(9n)) {
  border-right: 3px solid var(--electric-blue);
}

.sudoku-cell:nth-child(n+19):nth-child(-n+27), 
.sudoku-cell:nth-child(n+46):nth-child(-n+54) { 
  border-bottom: 3px solid var(--electric-blue) !important;
}

.sudoku-cell:nth-child(n+28):nth-child(-n+36), 
.sudoku-cell:nth-child(n+55):nth-child(-n+63) { 
  border-top: 1px solid var(--internal-border-color);
}

.initial-number {
  background-color: var(--initial-cell-bg) !important;
  font-weight: bold;
  color: #a0a0a0;
  cursor: default;
}

.highlighted {
  background-color: #004d99; 
}

.same-number {
  background-color: #003366; 
}

.error {
  background-color: #e74c3c !important; 
  color: var(--white);
}

#number-selector {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  justify-content: center;
}

.number-btn {
  width: 45px;
  height: 45px;
  font-size: 1.2em;
  font-weight: bold;
  border-radius: 5px;
  border: 2px solid var(--electric-blue);
  background-color: var(--dark-blue);
  color: var(--white);
  cursor: pointer;
  transition: transform 0.2s ease, background-color 0.2s ease;
}

#clear-btn {
  width: auto; 
  min-width: 70px; 
  padding: 0 10px; 
  height: 45px;
  font-size: 1.2em;
  font-weight: bold;
  border-radius: 5px;
  border: 2px solid var(--electric-blue);
  background-color: var(--dark-blue);
  color: var(--white);
  cursor: pointer;
  transition: transform 0.2s ease, background-color 0.2s ease;
}

.number-btn:hover, #clear-btn:hover, #load-daily-sudoku-btn:hover {
  background-color: var(--electric-blue);
  transform: scale(1.05);
}

#controls {
  display: flex;
  gap: 20px;
  margin-top: 10px;
}

#check-btn, #new-game-btn, #load-daily-sudoku-btn {
  padding: 10px 20px;
  font-size: 1em;
  border-radius: 5px;
  border: none;
  background-color: var(--electric-blue);
  color: var(--white);
  cursor: pointer;
  transition: background-color 0.3s ease;
}

#check-btn:hover, #new-game-btn:hover {
  background-color: var(--dark-blue);
}

#game-status {
  margin-top: 20px;
  font-size: 1.1em;
  font-weight: bold;
  color: var(--electric-blue); 
  text-align: center;
}