.btn {
    font-weight: 600;
    border-radius: 0.5rem;
    transition: all 0.3s;
}
.primary-btn {
    background-color: #3B82F6;
    color: white;
}
.primary-btn:hover {
    background-color: #2563EB;
}
.secondary-btn {
    background-color: #10B981;
    color: white;
    padding: 10px 24px;
}
.secondary-btn:hover {
    background-color: #059669;
}
.math-display {
    font-size: 1.5rem;
    margin: 20px 0;
    text-align: center;
    background: white;
    padding: 15px;
    border-radius: 8px;
    border: 2px solid #E5E7EB;
}
.virtual-keyboard {
    background: #F3F4F6;
    padding: 10px;
    border-radius: 8px;
    margin: 15px 0;
}
.vk-btn {
    background: white;
    border: 1px solid #D1D5DB;
    border-radius: 4px;
    padding: 10px 0;
    font-size: 1.2rem;
    font-weight: bold;
    cursor: pointer;
    transition: background 0.2s;
    display: flex;
    justify-content: center;
    align-items: center;
}
.vk-btn:hover {
    background: #E5E7EB;
}
.vk-del {
    background: #FEE2E2;
    color: #DC2626;
}
.vk-del:hover {
    background: #FCA5A5;
}