body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: #1e1e1e;
    color: #f0f0f0;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
}

.container {
    background-color: #2d2d2d;
    padding: 2.5rem;
    border-radius: 12px;
    box-shadow: 0 8px 16px rgba(0,0,0,0.3);
    text-align: center;
    max-width: 500px;
    width: 90%;
}

h1 {
    margin-bottom: 0.2rem;
    color: #61dafb;
}

.subtitle {
    margin-top: 0;
    color: #aaaaaa;
    margin-bottom: 1.5rem;
}

.difficulty-selector {
    margin-bottom: 1.5rem;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
}

.difficulty-selector label {
    font-weight: 600;
    color: #aaaaaa;
}

.difficulty-selector select {
    background-color: #1a1a1a;
    color: #f0f0f0;
    border: 1px solid #444;
    padding: 8px 12px;
    border-radius: 6px;
    font-size: 0.95rem;
    cursor: pointer;
}

.difficulty-selector select:focus {
    outline: none;
    border-color: #61dafb;
}

.math-expression {
    font-size: 2rem;
    font-weight: bold;
    margin: 1.5rem 0;
    padding: 1.5rem;
    background-color: #1a1a1a;
    border-radius: 8px;
    letter-spacing: 2px;
    border: 1px solid #333;
}

.solution-text {
    color: #4caf50;
    background-color: #1a2e1a;
    border-color: #2e5c2e;
}

.controls {
    display: flex;
    justify-content: space-between;
    gap: 1rem;
    margin-top: 1.5rem;
}

button {
    padding: 12px 20px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    border: none;
    border-radius: 6px;
    transition: all 0.3s ease;
    flex: 1;
}

.btn-primary {
    background-color: #007acc;
    color: white;
}

.btn-primary:hover {
    background-color: #005f9e;
}

.btn-secondary {
    background-color: #555555;
    color: white;
}

.btn-secondary:hover {
    background-color: #666666;
}

.hidden {
    display: none !important;
}

#solution-container {
    margin-top: 2rem;
    padding-top: 1.5rem;
    border-top: 1px solid #444;
}