:root {
    --primary-color: #0056b3;
    --secondary-color: #007bff;
    --bg-color: #f8f9fa;
    --card-bg-color: #ffffff;
    --text-color: #343a40;
    --light-text-color: #6c757d;
    --success-color: #28a745;
    --border-radius: 12px;
    --shadow: 0 4px 6px rgba(0, 0, 0, 0.05), 0 1px 3px rgba(0, 0, 0, 0.08);
}

* { margin: 0; padding: 0; box-sizing: border-box; }

body { font-family: 'Poppins', sans-serif; background-color: var(--bg-color); color: var(--text-color); line-height: 1.6; }

.page-header { max-width: 700px; margin: 0 auto; padding: 1.5rem 2rem 0; }
.btn-back { display: inline-block; padding: 0.5rem 1rem; background-color: #e9ecef; color: var(--primary-color); text-decoration: none; border-radius: var(--border-radius); font-weight: 600; transition: background-color 0.3s ease; }
.btn-back:hover { background-color: #dee2e6; }

.container { max-width: 700px; margin: 1rem auto 2rem; padding: 2.5rem; background-color: var(--card-bg-color); border-radius: var(--border-radius); box-shadow: var(--shadow); border: 1px solid #e9ecef; }
.calculator-header { text-align: center; margin-bottom: 2rem; }
.calculator-header h1 { color: var(--primary-color); margin-bottom: 0.5rem; }

.proportion-type {
    display: flex;
    justify-content: center;
    gap: 2rem;
    margin-bottom: 2rem;
}

.radio-label {
    display: flex;
    align-items: center;
    cursor: pointer;
    font-size: 1.1rem;
    font-weight: 600;
}
.radio-label input {
    margin-right: 0.5rem;
    width: 1.2em;
    height: 1.2em;
    accent-color: var(--primary-color);
}

.proportion-grid {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    gap: 1rem;
    align-items: center;
    text-align: center;
}

.proportion-grid input[type="number"] {
    width: 100%;
    padding: 0.85rem 1rem;
    border: 1px solid #ced4da;
    border-radius: var(--border-radius);
    font-size: 1.2rem;
    font-family: 'Poppins', sans-serif;
    text-align: center;
}
.proportion-grid input[type="number"]:focus {
    outline: none;
    border-color: var(--secondary-color);
    box-shadow: 0 0 0 3px rgba(0, 123, 255, 0.25);
}

.grid-text {
    font-size: 1.1rem;
    color: var(--light-text-color);
}

.result-box-x {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 100%;
    height: 100%;
    background-color: #e9ecef;
    border: 1px dashed #ced4da;
    border-radius: var(--border-radius);
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary-color);
}

#results-container { margin-top: 2.5rem; }
#results-container.hidden { display: none; }
.result-display {
    padding: 1.5rem;
    background-color: #e7f5ff;
    border-left: 5px solid var(--success-color);
    font-size: 1.3rem;
    font-weight: 600;
    text-align: center;
    border-radius: var(--border-radius);
}

.explanation-article { max-width: 700px; margin: 0 auto 2rem; padding: 2.5rem; background-color: var(--card-bg-color); border-radius: var(--border-radius); box-shadow: var(--shadow); border: 1px solid #e9ecef; }
.explanation-article h2, .explanation-article h3 { color: var(--primary-color); margin-bottom: 1rem; }
.explanation-article h3 { margin-top: 1.5rem; border-bottom: 1px solid #eee; padding-bottom: 0.5rem; font-size: 1.3rem; }
.explanation-article p, .explanation-article ul, .explanation-article ol { margin-bottom: 1rem; }
.explanation-article ul, .explanation-article ol { padding-left: 25px; }
.explanation-article .formula { text-align: center; font-size: 1.2rem; font-weight: 600; background-color: #f1f3f5; padding: 1rem; border-radius: var(--border-radius); }

.main-footer { text-align: center; padding: 2rem 1rem; color: var(--light-text-color); font-size: 0.9rem; }

@media (max-width: 768px) {
    .container, .page-header, .explanation-article { margin-left: 1rem; margin-right: 1rem; padding: 1.5rem; }
}