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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #0f4c81 0%, #11998e 100%);
    min-height: 100vh;
    padding: 20px;
}

.app-container {
    max-width: 760px;
    margin: 0 auto;
    background-color: #ffffff;
    padding: 30px;
    border-radius: 18px;
    box-shadow: 0 12px 36px rgba(0, 0, 0, 0.22);
}

.back-button {
    display: inline-block;
    margin-bottom: 20px;
    padding: 10px 18px;
    background-color: #0f4c81;
    color: #ffffff;
    text-decoration: none;
    border-radius: 8px;
    transition: all 0.2s;
}

.back-button:hover {
    background-color: #0b3a62;
    transform: translateY(-1px);
}

.preview-btn {
    display: none;
    margin: 0 auto 20px;
    padding: 10px 20px;
    background-color: #11998e;
    color: #ffffff;
    border: none;
    border-radius: 8px;
    font-size: 1em;
    cursor: pointer;
    transition: all 0.2s;
}

.preview-btn.visible {
    display: block;
}

.preview-btn:hover {
    background-color: #0f8177;
    transform: translateY(-1px);
}

h1 {
    text-align: center;
    color: #0f4c81;
    margin-bottom: 8px;
    font-size: 2em;
}

.subtitle {
    text-align: center;
    color: #4f6373;
    margin-bottom: 28px;
}

#question-counter {
    text-align: center;
    color: #4f6373;
    margin-bottom: 12px;
    font-size: 1.05em;
}

#diagram-container {
    display: none;
    margin: 0 auto 16px;
    padding: 10px;
    border: 2px solid #d8e4ee;
    border-radius: 12px;
    background: #f7fbff;
}

#diagram-container.has-diagram {
    display: block;
}

#diagram-container svg {
    width: 100%;
    max-width: 560px;
    height: auto;
    display: block;
    margin: 0 auto;
}

.diagram-caption {
    margin-top: 8px;
    text-align: center;
    color: #355166;
    font-size: 0.95em;
}

#question {
    color: #12212c;
    margin-bottom: 24px;
    line-height: 1.5;
    font-size: 1.3em;
}

.answer-row {
    display: grid;
    gap: 10px;
    margin-bottom: 16px;
}

.answer-label {
    font-weight: 600;
    color: #1f2f3b;
}

#answer-input {
    padding: 12px;
    border: 2px solid #b9c8d4;
    border-radius: 8px;
    font-size: 1.05em;
}

#answer-input:focus {
    outline: none;
    border-color: #0f4c81;
    box-shadow: 0 0 0 3px rgba(15, 76, 129, 0.15);
}

#check-btn,
#next-btn,
#restart-btn {
    padding: 12px 16px;
    border: none;
    border-radius: 8px;
    color: #ffffff;
    font-size: 1em;
    font-weight: 700;
    cursor: pointer;
    transition: transform 0.2s, filter 0.2s;
}

#check-btn {
    background-color: #0f4c81;
}

#next-btn,
#restart-btn {
    width: 100%;
    background-color: #11998e;
    margin-top: 6px;
}

#check-btn:hover,
#next-btn:hover,
#restart-btn:hover {
    transform: translateY(-1px);
    filter: brightness(1.06);
}

#feedback {
    padding: 12px;
    border-radius: 8px;
    margin-bottom: 10px;
    display: none;
    line-height: 1.4;
}

#feedback.correct {
    display: block;
    background-color: #d9f4e9;
    color: #12623f;
    border: 1px solid #2f9c67;
}

#feedback.incorrect {
    display: block;
    background-color: #fde2e2;
    color: #8e1f1f;
    border: 1px solid #d14b4b;
}

#score-container {
    text-align: center;
}

#score-container h2 {
    color: #0f4c81;
    margin-bottom: 12px;
}

#score-container p {
    color: #1f2f3b;
    margin-bottom: 16px;
    line-height: 1.5;
}

#preview-container h2 {
    color: #0f4c81;
    margin-bottom: 16px;
    text-align: center;
}

#preview-navigation {
    display: flex;
    gap: 12px;
    justify-content: center;
    align-items: center;
    margin-bottom: 24px;
    flex-wrap: wrap;
}

#preview-navigation button {
    padding: 10px 16px;
    border: none;
    border-radius: 8px;
    background-color: #0f4c81;
    color: #ffffff;
    font-size: 0.95em;
    cursor: pointer;
    transition: all 0.2s;
}

#preview-navigation button:hover:not(:disabled) {
    background-color: #0b3a62;
    transform: translateY(-1px);
}

#preview-navigation button:disabled {
    background-color: #b9c8d4;
    cursor: not-allowed;
}

#exit-preview-btn {
    background-color: #11998e !important;
}

#exit-preview-btn:hover {
    background-color: #0f8177 !important;
}

#preview-counter {
    color: #0f4c81;
    font-weight: 600;
    font-size: 1.05em;
}

#preview-question-display {
    padding: 20px;
    background-color: #f7fbff;
    border: 2px solid #d8e4ee;
    border-radius: 12px;
    line-height: 1.6;
}

.preview-question-text {
    color: #12212c;
    font-size: 1.2em;
    margin-bottom: 20px;
    font-weight: 500;
}

.preview-answer-box {
    background-color: #d9f4e9;
    border: 1px solid #2f9c67;
    border-radius: 8px;
    padding: 12px;
    margin-top: 16px;
}

.preview-answer-box p {
    margin: 6px 0;
    color: #12623f;
}

.preview-answer-box strong {
    color: #0f4c81;
}

@media (max-width: 640px) {
    .app-container {
        padding: 20px;
    }

    h1 {
        font-size: 1.6em;
    }

    #question {
        font-size: 1.15em;
    }

    #diagram-container {
        padding: 8px;
    }
}