body {
    background: radial-gradient(circle at top, #0f2027, #203a43, #2c5364);
    color: #fff;
    font-family: 'Orbitron', sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
}

.calculator {
    background: rgba(15,20,30, 0.9);
    padding: 20px;
    border: 2px solid #0ff;
    border-radius: 20px;
    box-shadow: 0 0 30px #0ff, inset 0 0 20px rgba(0,255,255,0.2);
    width: 340px;
    backdrop-filter: blur(6px);
}

#display {
    text-align: right;
    margin-bottom: 20px;
    padding: 15px;
    border: 1px solid #0ff;
    border-radius: 11px;
    background: rgba(0,0,0, 0.6);
    box-shadow: inset 0 0 10px rgba(0, 255, 255, 0.5);
}

#expression {
    font-size: 1.2em;
    color: #00e0ff;
    min-height: 23px;
    opacity: 0.8;
}

#result {
    font-size: 2em;
    font-weight: bold;
    color: #0ff;
    text-shadow: 0 0 11px #0ff;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    /* ще се повтарят по четири на ред */
    gap: 11px;
}

.btn {
    padding: 18px;
    font-size: 1.2em;
    font-family: inherit;
    border: none;
    border-radius: 11px;
    cursor: pointer;
    color: #0ff;
    background: linear-gradient(145deg, #0f2027, #203a43);
    box-shadow: 0 0 11px rgba(0,255, 255, 0.3),
                inset 0 0 8px rgba(0, 255, 255, 0.2);
    transition: all 0.2s ease-in-out;
}

.btn:hover {
    background: linear-gradient(145deg, #203a43, #2c5364);
    color: #fff;
    box-shadow: 0 0 20px #0ff, inset 0 0 12px rgba(0,255,255,0.6);
}
.zero { 
    grid-column: span 2; /* Making the button wider (2 spans) */
}

.plus{
    grid-row: span 2; /* Making the button taller (2 spans) */
    padding: 18px;
    font-size: 1.2em;
    font-family: inherit;
    border: none;
    border-radius: 11px;
    cursor: pointer;
    color: #0ff;
    background: linear-gradient(145deg, #0f2027, #203a43);
    box-shadow: 0 0 11px rgba(0,255, 255, 0.3),
                inset 0 0 8px rgba(0, 255, 255, 0.2);
    transition: all 0.2s ease-in-out;
}

.plus:hover {
    background: linear-gradient(145deg, #203a43, #2c5364);
    color: #fff;
    box-shadow: 0 0 20px #0ff, inset 0 0 12px rgba(0,255,255,0.6);
}
.equal {
    grid-column: span 3;
    background: linear-gradient(180deg, #0ff, #333399);
    color: #fff;
    box-shadow: 0 0 15px #067ae0, inset 0 0 8px rgba(0, 217, 255, 0.6);
}
.equal:hover {
    background: linear-gradient(145deg,  #00ffff, #0099ff);
    box-shadow: 0 0 25px #00ffff, inset 0 0 11px rgba(0, 255, 204, 0.6);
}

.signature {
    margin-top: 25px;
    color: #66ccff;
    text-align: center;
    font-size: 0.85rem;
    font-style: italic;
    font-weight: 10;
    font-family: 'Orbitron', sans-serif;
    letter-spacing: 1px;
    opacity: 0.5;
}

.signature span {
    color: #00ffea;
    font-weight: bold;
}