* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    -webkit-tap-highlight-color: transparent; /* remove blue tap flash (mobile) */
}

body{
    font-family: 'Poppins', sans-serif;
    height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    background-color: black;
    color: white;
    transition: background-color 0.15s ease;
}

h1 {
    font-size: 2rem;
    margin-bottom: 0.5rem;
    font-weight: 700;
    letter-spacing: 0.5px;
}

#high-score {
  font-size: 1rem;
  font-weight: 500;
  opacity: 0.8;
  margin-bottom: 0.8rem;
}

.instruction {
  font-size: 0.95rem;
  opacity: 0.7;
}

h4 {
    margin-bottom: 1.5rem;
}

.btn-container{
    /* Mobile → 80vw
    Desktop → capped at 320px */
    width: 80vw;
    max-width: 320px;
    aspect-ratio: 1 / 1; /* Keeps width and height equal on all screens */

    display: grid;
    grid-template-columns: 1fr 1fr;
    grid-template-rows: 1fr 1fr;

    gap: 12px;
    padding: 10px;
    border-radius: 50%;

    position: relative; /* Needed so center circle is positioned correctly */
    touch-action: none;
}

.btn{
    cursor: pointer;
    transition: box-shadow 0.15s ease, transform 0.1s ease;
}

.green { 
    background-color: #2ecc71; 
    border-top-left-radius: 100%;
}
.red { 
    background-color: #ff4d4d; 
    border-top-right-radius: 100%;
}
.yellow { 
    background-color: #f1c40f; 
    border-bottom-left-radius: 100%;
}
.blue { 
    background-color: #3498db; 
    border-bottom-right-radius: 100%;
}

.center{
    font-weight: 600;

    position: absolute; /* for place it at center according to parent */
    /* Size relative to board, scales automatically */
    height: 35%;
    width: 35%;

    /* Always center element regardless of its size */
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);

    background-color: black;
    border-radius: 50%;

    /* this four for inner content of circle  */
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;

    text-align: center;
    line-height: 1.4;
}

.green.flash {
    box-shadow: 0 0 30px #2ecc71;
}

.red.flash {
    box-shadow: 0 0 30px #ff4d4d;
}

.yellow.flash {
    box-shadow: 0 0 30px #f1c40f;
}

.blue.flash {
    box-shadow: 0 0 30px #3498db;
}

.btn.flash {
  transform: scale(1.03);
}