:root {
    --bg-color: #0b0d14;
    --card-bg: rgba(22, 27, 38, 0.75);
    --input-bg: rgba(14, 18, 27, 0.85);
    --border-color: rgba(255, 255, 255, 0.08);
    --border-highlight: rgba(0, 242, 254, 0.4);
    --text-main: #f3f4f6;
    --text-muted: #9ca3af;
    --text-sub: #6b7280;
    --accent-color: #00f2fe;
    --accent-gradient: linear-gradient(135deg, #00f2fe 0%, #4facfe 100%);
    --accent-glow: rgba(0, 242, 254, 0.25);
    --shadow-main: 0 20px 50px rgba(0, 0, 0, 0.5);
    --shadow-card: 0 8px 32px 0 rgba(0, 0, 0, 0.37);
    --radius-lg: 20px;
    --radius-md: 12px;
    --radius-sm: 8px;
    --transition-fast: 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
    font-family: 'Inter', 'Noto Sans JP', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background: radial-gradient(circle at 50% 0%, #151d30 0%, #0b0d14 70%);
    color: var(--text-main);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    padding: 1.5rem 1rem;
    line-height: 1.5;
}

.converter-wrapper {
    width: 100%;
    max-width: 680px;
    background: var(--card-bg);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-main);
    overflow: hidden;
    position: relative;
}

.converter-wrapper::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 3px;
    background: var(--accent-gradient);
}

header {
    padding: 2.2rem 2rem 1.5rem;
    text-align: center;
    position: relative;
}

.lang-switch {
    position: absolute;
    top: 2rem;
    right: 2rem;
    display: flex;
    align-items: center;
    gap: 0.15rem;
    background: rgba(10, 13, 20, 0.6);
    border: 1px solid var(--border-color);
    border-radius: 20px;
    padding: 0.2rem 0.45rem;
}

.lang-btn {
    background: transparent;
    border: none;
    color: var(--text-muted);
    font-size: 0.75rem;
    font-weight: 600;
    cursor: pointer;
    padding: 0.2rem 0.45rem;
    border-radius: 12px;
    transition: all var(--transition-fast);
}

.lang-btn:hover {
    color: var(--text-main);
}

.lang-btn.active {
    color: #0b0d14;
    background: var(--accent-gradient);
    font-weight: 700;
}

.lang-divider {
    color: var(--text-sub);
    font-size: 0.7rem;
    user-select: none;
}

.header-badge {
    display: inline-block;
    padding: 0.25rem 0.75rem;
    font-size: 0.75rem;
    font-weight: 600;
    letter-spacing: 1px;
    color: var(--accent-color);
    background: rgba(0, 242, 254, 0.1);
    border: 1px solid rgba(0, 242, 254, 0.25);
    border-radius: 20px;
    margin-bottom: 0.75rem;
    text-transform: uppercase;
}

header h1 {
    font-size: 2rem;
    font-weight: 700;
    background: linear-gradient(180deg, #ffffff 0%, #d1d5db 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    letter-spacing: -0.5px;
}

header .subtitle {
    font-size: 0.9rem;
    color: var(--text-muted);
    margin-top: 0.4rem;
}

main {
    padding: 0 2rem 2rem;
}

/* Category Tabs */
.category-tabs {
    display: flex;
    gap: 0.5rem;
    margin-bottom: 2rem;
    background: rgba(10, 13, 20, 0.6);
    padding: 0.4rem;
    border-radius: var(--radius-md);
    border: 1px solid var(--border-color);
}

.category-tabs button {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 0.7rem 0.4rem;
    background: transparent;
    border: none;
    border-radius: var(--radius-sm);
    color: var(--text-muted);
    cursor: pointer;
    transition: all var(--transition-fast);
    outline: none;
}

.category-tabs button .tab-jp {
    font-size: 0.9rem;
    font-weight: 600;
}

.category-tabs button .tab-en {
    font-size: 0.7rem;
    opacity: 0.7;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-top: 2px;
}

.category-tabs button .tab-en:empty {
    display: none;
}

.category-tabs button:hover {
    color: var(--text-main);
    background: rgba(255, 255, 255, 0.05);
}

.category-tabs button.active {
    background: var(--accent-gradient);
    color: #0b0d14;
    box-shadow: 0 4px 15px var(--accent-glow);
}

.category-tabs button.active .tab-jp,
.category-tabs button.active .tab-en {
    opacity: 1;
    font-weight: 700;
}

/* Converter Body */
.converter-body {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    margin-bottom: 1.5rem;
}

.converter-group {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 0.6rem;
}

.group-header label {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--text-main);
    display: flex;
    align-items: center;
    gap: 0.4rem;
}

.group-header .country-sub {
    font-size: 0.75rem;
    font-weight: 400;
    color: var(--text-muted);
}

.input-wrapper {
    position: relative;
}

input[type="number"],
select {
    width: 100%;
    background-color: var(--input-bg);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    color: var(--text-main);
    font-family: inherit;
    font-size: 1.25rem;
    font-weight: 500;
    padding: 0.85rem 1rem;
    transition: all var(--transition-fast);
    -webkit-appearance: none;
    -moz-appearance: none;
    appearance: none;
}

input[type="number"] {
    font-size: 1.4rem;
    font-weight: 600;
}

input[type="number"]::placeholder {
    color: var(--text-sub);
}

select {
    font-size: 0.95rem;
    cursor: pointer;
    background-image: url("data:image/svg+xml;charset=UTF-8,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%2300f2fe' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3e%3cpolyline points='6 9 12 15 18 9'%3e%3c/polyline%3e%3c/svg%3e");
    background-repeat: no-repeat;
    background-position: right 0.9rem center;
    background-size: 1.1em;
    padding-right: 2.5rem;
}

select option {
    background-color: #12151e;
    color: var(--text-main);
    padding: 0.5rem;
}

input[type="number"]:focus,
select:focus {
    outline: none;
    border-color: var(--accent-color);
    box-shadow: 0 0 0 3px var(--accent-glow);
}

/* Swap Button */
.swap-container {
    display: flex;
    align-items: center;
    justify-content: center;
    padding-top: 1.5rem;
}

.swap-button {
    width: 44px;
    height: 44px;
    border-radius: 50%;
    background: rgba(255, 255, 255, 0.05);
    border: 1px solid var(--border-color);
    color: var(--accent-color);
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: all var(--transition-fast);
    flex-shrink: 0;
}

.swap-button:hover {
    background: var(--accent-gradient);
    color: #0b0d14;
    border-color: transparent;
    transform: rotate(180deg) scale(1.05);
    box-shadow: 0 0 15px var(--accent-glow);
}

.swap-button:active {
    transform: rotate(180deg) scale(0.95);
}

/* Formula Card */
.formula-card {
    display: flex;
    align-items: center;
    gap: 0.8rem;
    background: rgba(0, 242, 254, 0.04);
    border: 1px dashed rgba(0, 242, 254, 0.2);
    border-radius: var(--radius-md);
    padding: 0.85rem 1.2rem;
    margin-top: 1rem;
}

.formula-icon {
    font-size: 1.2rem;
}

.formula-content {
    display: flex;
    flex-direction: column;
    gap: 0.1rem;
}

.formula-label {
    font-size: 0.72rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.formula-text {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--accent-color);
    font-family: monospace, 'Courier New', Courier;
}

footer {
    text-align: center;
    padding: 1.2rem;
    font-size: 0.8rem;
    color: var(--text-sub);
    border-top: 1px solid var(--border-color);
}

/* Responsive Design */
@media (max-width: 600px) {
    body {
        padding: 0;
        align-items: flex-start;
    }

    .converter-wrapper {
        border-radius: 0;
        min-height: 100vh;
        border: none;
    }

    header {
        padding: 1.8rem 1.2rem 1.2rem;
    }

    .lang-switch {
        top: 1.2rem;
        right: 1.2rem;
        padding: 0.15rem 0.35rem;
    }

    .lang-btn {
        font-size: 0.7rem;
        padding: 0.15rem 0.35rem;
    }

    header h1 {
        font-size: 1.6rem;
    }

    main {
        padding: 0 1.2rem 1.5rem;
    }

    .category-tabs {
        gap: 0.25rem;
        padding: 0.3rem;
    }

    .category-tabs button {
        padding: 0.5rem 0.2rem;
    }

    .category-tabs button .tab-jp {
        font-size: 0.8rem;
    }

    .category-tabs button .tab-en {
        font-size: 0.65rem;
    }

    .converter-body {
        flex-direction: column;
        gap: 0.8rem;
    }

    .swap-container {
        padding-top: 0;
        margin: -0.2rem 0;
    }

    .swap-button svg {
        transform: rotate(90deg);
    }

    .swap-button:hover svg {
        transform: rotate(270deg);
    }
}