/* ===== CSS Variables & Reset ===== */
:root {
    /* Dark mode color palette - minimal accents */
    --color-bg: #0a0a0a;
    --color-bg-elevated: #151515;
    --color-bg-card: #1a1a1a;
    --color-bg-hover: #222222;
    --color-border: #2a2a2a;
    --color-border-focus: #3a3a3a;

    --color-text-primary: #e8e8e8;
    --color-text-secondary: #a0a0a0;
    --color-text-tertiary: #707070;

    /* Minimal accent - subtle blue-gray */
    --color-accent: #4a5568;
    --color-accent-hover: #5a6578;
    --color-accent-light: #606f88;

    /* Unit Colors */
    --color-length: #b8922260;
    --color-length-dk: #b8922220;
    --color-area: #32a27f50;
    --color-area-dk: #32a27f20;
    --color-mass: #a684ff50;
    --color-mass-dk: #a684ff20;
    --color-temperature: #ff646750;
    --color-temperature-dk: #ff646720;
    --color-time: #54eafd50;
    --color-time-dk: #54eafd20;
    --color-volume: #fd901250;
    --color-volume-dk: #fd901220;
    --color-speed: #36384f;
    --color-speed-dk: #15151c;

    /* Spacing system */
    --space-xs: 0.5rem;
    --space-sm: 0.75rem;
    --space-md: 1rem;
    --space-lg: 1.5rem;
    --space-xl: 2rem;
    --space-2xl: 3rem;

    /* Typography */
    --font-sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    --font-mono: "SF Mono", Monaco, "Cascadia Code", "Roboto Mono", Consolas, monospace;

    /* Border radius */
    --radius-sm: 0.375rem;
    --radius-md: 0.5rem;
    --radius-lg: 0.75rem;
    --radius-xl: 1rem;

    /* Transitions */
    --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-base: 200ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-slow: 300ms cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
    font-family: var(--font-sans);
    background-color: var(--color-bg);
    color: var(--color-text-primary);
    line-height: 1.6;
    min-height: 100vh;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

/* ===== Layout ===== */
.app-container {
    max-width: 800px;
    margin: 0 auto;
    padding: var(--space-lg);
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

.app-header {
    text-align: center;
    padding: var(--space-xl) 0;
    margin-bottom: var(--space-xl);
}

.app-title {
    font-size: 2rem;
    font-weight: 600;
    letter-spacing: -0.02em;
    margin-bottom: var(--space-xs);
}

.app-subtitle {
    color: var(--color-text-secondary);
    font-size: 0.875rem;
    font-weight: 400;
}

.app-main {
    flex: 1;
}

.app-footer {
    margin-top: var(--space-2xl);
    padding-top: var(--space-xl);
    text-align: center;
    color: var(--color-text-tertiary);
    font-size: 0.875rem;
}

/* ===== Views ===== */
.view {
    display: none;
    animation: fadeIn var(--transition-base);
}

.view--active {
    display: block;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(8px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* ===== Category Grid ===== */
.category-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
    gap: var(--space-md);
    padding: var(--space-md) 0;
}

.category-card {
    background-color: var(--color-bg-card);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-lg);
    padding: var(--space-xl) var(--space-lg);
    cursor: pointer;
    transition: all var(--transition-base);
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--space-sm);
    font-family: var(--font-sans);
    color: var(--color-text-primary);
    font-size: 1rem;
}

.card-length {
    background-color: var(--color-length-dk);
    color: var(--color-text-primary);
    border-color: var(--color-length);
}

.card-mass {
    background-color: var(--color-mass-dk);
    color: var(--color-text-primary);
    border-color: var(--color-mass);
}

.card-time {
    background-color: var(--color-time-dk);
    color: var(--color-text-primary);
    border-color: var(--color-time);
}

.card-temperature {
    background-color: var(--color-temperature-dk);
    color: var(--color-text-primary);
    border-color: var(--color-temperature);
}

.card-area {
    background-color: var(--color-area-dk);
    color: var(--color-text-primary);
    border-color: var(--color-area);
}

.card-volume {
    background-color: var(--color-volume-dk);
    color: var(--color-text-primary);
    border-color: var(--color-volume);
}

.card-speed {
    background-color: var(--color-speed-dk);
    color: var(--color-text-primary);
    border-color: var(--color-speed);
}

.category-card:hover {
    background-color: var(--color-bg-hover);
    border-color: var(--color-border-focus);
    transform: translateY(-2px);
}

.category-card:active {
    transform: translateY(0);
}

.category-icon {
    font-size: 2rem;
    line-height: 1;
    opacity: 0.9;
    img {
        width: 24px;
        height: 24px;
        filter: brightness(0) saturate(100%) invert(80%);
    }
}

.category-name {
    font-weight: 500;
    letter-spacing: -0.01em;
}

/* ===== Converter View ===== */
.converter-view {
    max-width: 600px;
    margin: 0 auto;
}

.back-button {
    background-color: transparent;
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
    padding: var(--space-sm) var(--space-md);
    color: var(--color-text-secondary);
    font-family: var(--font-sans);
    font-size: 0.875rem;
    cursor: pointer;
    transition: all var(--transition-fast);
    display: inline-flex;
    align-items: center;
    gap: var(--space-xs);
    margin-bottom: var(--space-lg);
}

.back-button:hover {
    background-color: var(--color-bg-card);
    border-color: var(--color-border-focus);
    color: var(--color-text-primary);
}

.converter-title {
    font-size: 1.5rem;
    font-weight: 600;
    letter-spacing: -0.02em;
    margin-bottom: var(--space-xl);
}

/* ===== Forms ===== */
.converter-form {
    display: flex;
    flex-direction: column;
    gap: var(--space-lg);
}

.input-group {
    display: flex;
    flex-direction: column;
    gap: var(--space-xs);
}

.input-label {
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--color-text-secondary);
    letter-spacing: -0.01em;
}

.input-field {
    background-color: var(--color-bg-card);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
    padding: var(--space-md);
    color: var(--color-text-primary);
    font-family: var(--font-mono);
    font-size: 1.125rem;
    transition: all var(--transition-fast);
    width: 100%;
}

.input-field::placeholder {
    color: var(--color-text-tertiary);
}

.input-field:focus {
    outline: none;
    border-color: var(--color-accent);
    background-color: var(--color-bg-elevated);
}

/* Remove number input arrows */
.input-field::-webkit-outer-spin-button,
.input-field::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}

.input-field[type="number"] {
    -moz-appearance: textfield;
}

/* ===== Conversion Controls ===== */
.conversion-controls {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    gap: var(--space-md);
    align-items: end;
}

.select-group {
    display: flex;
    flex-direction: column;
    gap: var(--space-xs);
}

.select-label {
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--color-text-secondary);
    letter-spacing: -0.01em;
}

.select-field {
    background-color: var(--color-bg-card);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-md);
    padding: var(--space-md);
    color: var(--color-text-primary);
    font-family: var(--font-sans);
    font-size: 0.9375rem;
    cursor: pointer;
    transition: all var(--transition-fast);
    width: 100%;
    appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='16' height='16' viewBox='0 0 16 16'%3E%3Cpath fill='%23a0a0a0' d='M4.427 6.427l3.396 3.396a.25.25 0 00.354 0l3.396-3.396A.25.25 0 0011.396 6H4.604a.25.25 0 00-.177.427z'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right var(--space-md) center;
    padding-right: calc(var(--space-md) + 24px);
}

.select-field:focus {
    outline: none;
    border-color: var(--color-accent);
    background-color: var(--color-bg-elevated);
}

.select-field:hover {
    border-color: var(--color-border-focus);
}

.conversion-arrow {
    color: var(--color-text-tertiary);
    font-size: 1.5rem;
    padding-bottom: var(--space-md);
    text-align: center;
}

/* ===== Buttons ===== */
.convert-button {
    background-color: var(--color-accent);
    border: 1px solid var(--color-accent);
    border-radius: var(--radius-md);
    padding: var(--space-md) var(--space-xl);
    color: var(--color-text-primary);
    font-family: var(--font-sans);
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition-fast);
    letter-spacing: -0.01em;
}

.convert-length {
    background-color: var(--color-length);
    border-color: var(--color-length-dk);
}

.convert-area {
    background-color: var(--color-area);
    border-color: var(--color-area-dk);
}

.convert-mass {
    background-color: var(--color-mass);
    border-color: var(--color-mass-dk);
}

.convert-time {
    background-color: var(--color-time);
    border-color: var(--color-time-dk);
}

.convert-volume {
    background-color: var(--color-volume);
    border-color: var(--color-volume-dk);
}

.convert-speed {
    background-color: var(--color-speed);
    border-color: var(--color-speed-dk);
}

.convert-temperature {
    background-color: var(--color-temperature);
    border-color: var(--color-temperature-dk);
}

.convert-button:hover {
    background-color: var(--color-accent-hover);
    border-color: var(--color-accent-hover);
    transform: translateY(-1px);
}

.convert-button:active {
    transform: translateY(0);
}

/* ===== Result Display ===== */
.result-display {
    margin-top: var(--space-xl);
    padding: var(--space-lg);
    background-color: var(--color-bg-elevated);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-lg);
    min-height: 80px;
    display: flex;
    align-items: center;
    justify-content: center;
    text-align: center;
    color: var(--color-text-secondary);
    font-size: 1rem;
    line-height: 1.6;
    transition: all var(--transition-base);
}

.result-display:not(:empty) {
    color: var(--color-text-primary);
    background-color: var(--color-bg-card);
    border-color: var(--color-border-focus);
    font-family: var(--font-mono);
    font-size: 1.0625rem;
}

/* ===== Responsive Design ===== */
@media (max-width: 640px) {
    .app-container {
        padding: var(--space-md);
    }

    .app-title {
        font-size: 1.75rem;
    }

    .category-grid {
        grid-template-columns: repeat(2, 1fr);
        gap: var(--space-sm);
    }

    .category-card {
        padding: var(--space-lg) var(--space-md);
    }

    .category-icon {
        font-size: 1.75rem;
    }

    .conversion-controls {
        grid-template-columns: 1fr;
        gap: var(--space-md);
    }

    .conversion-arrow {
        transform: rotate(90deg);
        padding: 0;
        margin: calc(var(--space-xs) * -1) 0;
    }

    .converter-title {
        font-size: 1.25rem;
    }
}

/* ===== Focus Visible (Accessibility) ===== */
*:focus-visible {
    outline: 2px solid var(--color-accent-light);
    outline-offset: 2px;
}

button:focus-visible,
.category-card:focus-visible {
    outline: 2px solid var(--color-accent-light);
    outline-offset: 2px;
}

/* ===== Selection ===== */
::selection {
    background-color: var(--color-accent);
    color: var(--color-text-primary);
}