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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.container {
    background: white;
    border-radius: 10px;
    padding: 30px;
    max-width: 600px;
    width: 100%;
    box-shadow: 0 10px 40px rgba(0, 0, 0, 0.2);
}

h1 {
    text-align: center;
    color: #333;
    margin-bottom: 30px;
    font-size: 24px;
}

.equations-display {
    background: linear-gradient(135deg, #f5f7fa 0%, #f8f9fa 100%);
    border: 2px solid #667eea;
    border-radius: 8px;
    padding: 20px;
    min-height: 300px;
    margin-bottom: 20px;
    font-size: 18px;
    font-family: 'Courier New', monospace;
    overflow-y: auto;
    max-height: 500px;
}

.equation-line {
    padding: 12px 0;
    line-height: 1.8;
    border-bottom: 1px solid #eee;
}

.equation-line:last-child {
    border-bottom: none;
}

.coefficient {
    color: #e74c3c;
    font-weight: bold;
}

.variable {
    color: #2980b9;
    font-weight: bold;
}

.term {
    display: inline;
}

/* Highlighted (newly added) terms */
.term-new {
    color: #e67e22;
    font-weight: bold;
    background: rgba(230, 126, 34, 0.12);
    border-radius: 3px;
    padding: 0 2px;
}

.equals {
    color: #555;
    font-weight: bold;
}

/* Verification lines */
.verify-line {
    background: #fffbf0;
}

.verify-label {
    color: #888;
    font-size: 14px;
    font-family: 'Segoe UI', sans-serif;
    letter-spacing: 0.5px;
}

.term.verify {
    color: #8e44ad;
}

.term.verify-ok {
    color: #27ae60;
    font-weight: bold;
}

.final-line {
    background: #f0fff4;
}

.checkmark {
    color: #27ae60;
    font-weight: bold;
    margin-left: 8px;
}

.first-line {
    font-weight: bold;
    font-size: 20px;
}

.controls {
    display: flex;
    gap: 10px;
    flex-direction: column;
    align-items: center;
}

.step-btn {
    padding: 12px 30px;
    font-size: 16px;
    font-weight: 600;
    border: none;
    border-radius: 8px;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    color: white;
    cursor: pointer;
    transition: all 0.3s;
    box-shadow: 0 4px 15px rgba(102, 126, 234, 0.4);
}

.step-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(102, 126, 234, 0.6);
}

.step-btn:active {
    transform: translateY(0);
}

.skip-btn {
    background: linear-gradient(135deg, #95a5a6 0%, #7f8c8d 100%);
    box-shadow: 0 4px 15px rgba(149, 165, 166, 0.4);
    font-size: 14px;
    padding: 8px 20px;
}

.skip-btn:hover {
    box-shadow: 0 6px 20px rgba(149, 165, 166, 0.6);
}

.difficulty-selector {
    display: flex;
    gap: 15px;
    justify-content: center;
}

.diff-btn {
    padding: 10px 20px;
    font-size: 14px;
    font-weight: 500;
    border: 2px solid #667eea;
    background: white;
    color: #667eea;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.3s;
}

.diff-btn:hover {
    background: #667eea;
    color: white;
}

/* Animation classes */
.fade-in {
    animation: fadeIn 0.7s ease-in;
}

.fade-out {
    animation: fadeOut 0.7s ease-out;
}

@keyframes fadeIn {
    from {
        opacity: 0;
    }
    to {
        opacity: 1;
    }
}

@keyframes fadeOut {
    from {
        opacity: 1;
    }
    to {
        opacity: 0;
    }
}

@keyframes blinkHighlight {
    0%   { opacity: 1; }
    15%  { opacity: 0.1; }
    30%  { opacity: 1; }
    45%  { opacity: 0.1; }
    60%  { opacity: 1; }
    75%  { opacity: 0.1; }
    100% { opacity: 1; }
}