:root {
    --primary-color: #0056b3;
    --secondary-color: #007bff;
    --bg-color: #f8f9fa;
    --card-bg-color: #ffffff;
    --text-color: #343a40;
    --light-text-color: #6c757d;
    --success-color: #28a745;
    --highlight-color: #e7f5ff;
    --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; }

.form-group { margin-bottom: 1.5rem; }
.form-group label { display: block; margin-bottom: 0.5rem; font-weight: 600; }
.form-group input { width: 100%; padding: 0.85rem 1rem; border: 1px solid #ced4da; border-radius: var(--border-radius); font-size: 1rem; font-family: 'Poppins', sans-serif; }
.form-group input:focus { outline: none; border-color: var(--secondary-color); box-shadow: 0 0 0 3px rgba(0, 123, 255, 0.25); }
.form-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 1.5rem; }

.radio-group { display: flex; flex-direction: column; gap: 0.75rem; }
.radio-label { display: flex; align-items: center; cursor: pointer; font-size: 1rem; }
.radio-label input { margin-right: 0.75rem; width: 1.2em; height: 1.2em; accent-color: var(--primary-color); }

button[type="submit"] { width: 100%; padding: 1rem; background: linear-gradient(90deg, var(--secondary-color) 0%, var(--primary-color) 100%); color: white; border: none; border-radius: var(--border-radius); font-size: 1.1rem; font-weight: 600; cursor: pointer; transition: transform 0.2s ease, box-shadow 0.3s ease; margin-top: 1rem; }
button[type="submit"]:hover { transform: translateY(-2px); box-shadow: 0 4px 10px rgba(0, 86, 179, 0.3); }

#results-container { margin-top: 2.5rem; padding-top: 1.5rem; border-top: 1px solid #eee; }
#results-container.hidden { display: none; }
#results-container h2 { text-align: center; margin-bottom: 1.5rem; color: var(--primary-color); }

.result-display { text-align: center; padding: 2rem; border-radius: var(--border-radius); background-color: var(--highlight-color); border: 2px solid var(--secondary-color); }
.result-display .label { font-size: 1.1rem; color: var(--primary-color); }
.result-display .btu-value { font-size: 3.5rem; font-weight: 700; display: block; line-height: 1.2; color: var(--text-color); }

.result-details { text-align: center; font-size: 0.9rem; color: var(--light-text-color); margin-top: 1rem; }

.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 { margin-bottom: 1rem; }
.explanation-article ul { padding-left: 25px; }

.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; }
    .form-grid { grid-template-columns: 1fr; gap: 0; }
}