body {
  text-align: center;
  background: radial-gradient(circle, #0f2027, #203a43, #2c5364);
  height: 100vh;
  margin: 0;
  padding: 0;
  font-family: "Poppins", sans-serif;
}

h1 {
  color: #fff;
  margin-top: 30px;
  font-size: 3rem;
}

h2 {
  color: #f8f8f8;
  margin-bottom: 10px;
  font-weight: 400;
}

/* Start Button */
#startBtn {
  padding: 12px 24px;
  font-size: 1.1rem;
  border-radius: 8px;
  border: none;
  cursor: pointer;
  background: #00ff99;
  color: #000;
  margin-bottom: 20px;
  transition: transform 0.2s ease, background 0.2s ease;
}

#startBtn:hover {
  transform: scale(1.05);
  background: #00e68a;
}

#startBtn:disabled {
  background: gray;
  cursor: not-allowed;
}

/* Progress bar */
#progressBar {
  width: 0;
  height: 10px;
  background: lime;
  border-radius: 5px;
  margin: 1rem auto;
  transition: width 0.4s ease;
}

/* Buttons */
.btn-container {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 20px;
}

.btn {
  height: 160px;
  width: 160px;
  border-radius: 20%;
  border: 8px solid #000;
  cursor: pointer;
  transition: all 0.2s ease-in-out;
}

.yellow { background-color: #f99b45; }
.red    { background-color: #d95980; }
.green  { background-color: #819ff9; }
.purple { background-color: #63aac0; }

.flash {
  box-shadow: 0 0 30px white;
  transform: scale(1.05);
}

.userFlash {
  box-shadow: 0 0 25px yellow;
  transform: scale(0.95);
}

/* Game over shake */
body.game-over {
  animation: shake 0.3s ease-in-out 0s 2;
}

@keyframes shake {
  0% { transform: translateX(0); }
  25% { transform: translateX(-10px); }
  50% { transform: translateX(10px); }
  75% { transform: translateX(-10px); }
  100% { transform: translateX(0); }
}