/* ===== Reset & Base ===== */
*,
*::before,
*::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

/* ===== Theme Variables ===== */
:root {
    --color-bg: #f8f9fa;
    --color-text: #2c3e50;
    --color-heading: #1a1a2e;
    --color-subtext: #5a6a7a;
    --color-card-bg: #fff;
    --color-card-border: #e8ecf1;
    --color-card-hover-border: #c4d0e0;
    --color-card-hover-shadow: rgba(0, 0, 0, 0.06);
    --color-card-active-shadow: rgba(0, 0, 0, 0.04);
    --color-link: #1a73e8;
    --color-link-hover: #1557b0;
    --color-link-active: #0d3f8f;
    --color-link-hover-bg: rgba(26, 115, 232, 0.08);
    --color-link-active-bg: rgba(26, 115, 232, 0.15);
    --color-mail: #188038;
    --color-mail-hover: #137333;
    --color-mail-active: #074e2b;
    --color-mail-hover-bg: rgba(24, 128, 56, 0.08);
    --color-mail-active-bg: rgba(14, 116, 110, 0.15);
    --color-card-title: #1a73e8;
    --color-card-desc: #8896a6;
    --color-footer-text: #8896a6;
    --color-footer-border: #e8ecf1;
    --color-footer-link: #8896a6;
    --color-footer-link-hover: #5a6a7a;
    --color-footer-link-hover-bg: rgba(0, 0, 0, 0.04);
    --color-separator: #c4d0e0;
}

/* System dark preference (no user override) */
@media (prefers-color-scheme: dark) {
    :root:not([data-theme="light"]) {
        --color-bg: #1a1a2e;
        --color-text: #d1d5db;
        --color-heading: #f0f0f5;
        --color-subtext: #9ca3af;
        --color-card-bg: #252540;
        --color-card-border: #3a3a5c;
        --color-card-hover-border: #5a5a8a;
        --color-card-hover-shadow: rgba(0, 0, 0, 0.3);
        --color-card-active-shadow: rgba(0, 0, 0, 0.2);
        --color-link: #6ea8fe;
        --color-link-hover: #9ec5fe;
        --color-link-active: #b6d4fe;
        --color-link-hover-bg: rgba(110, 168, 254, 0.12);
        --color-link-active-bg: rgba(110, 168, 254, 0.2);
        --color-mail: #4ade80;
        --color-mail-hover: #86efac;
        --color-mail-active: #bbf7d0;
        --color-mail-hover-bg: rgba(74, 222, 128, 0.12);
        --color-mail-active-bg: rgba(74, 222, 128, 0.2);
        --color-card-title: #6ea8fe;
        --color-card-desc: #9ca3af;
        --color-footer-text: #9ca3af;
        --color-footer-border: #3a3a5c;
        --color-footer-link: #9ca3af;
        --color-footer-link-hover: #d1d5db;
        --color-footer-link-hover-bg: rgba(255, 255, 255, 0.06);
        --color-separator: #5a5a8a;
    }
}

/* Explicit dark mode override */
[data-theme="dark"] {
    --color-bg: #1a1a2e;
    --color-text: #d1d5db;
    --color-heading: #f0f0f5;
    --color-subtext: #9ca3af;
    --color-card-bg: #252540;
    --color-card-border: #3a3a5c;
    --color-card-hover-border: #5a5a8a;
    --color-card-hover-shadow: rgba(0, 0, 0, 0.3);
    --color-card-active-shadow: rgba(0, 0, 0, 0.2);
    --color-link: #6ea8fe;
    --color-link-hover: #9ec5fe;
    --color-link-active: #b6d4fe;
    --color-link-hover-bg: rgba(110, 168, 254, 0.12);
    --color-link-active-bg: rgba(110, 168, 254, 0.2);
    --color-mail: #4ade80;
    --color-mail-hover: #86efac;
    --color-mail-active: #bbf7d0;
    --color-mail-hover-bg: rgba(74, 222, 128, 0.12);
    --color-mail-active-bg: rgba(74, 222, 128, 0.2);
    --color-card-title: #6ea8fe;
    --color-card-desc: #9ca3af;
    --color-footer-text: #9ca3af;
    --color-footer-border: #3a3a5c;
    --color-footer-link: #9ca3af;
    --color-footer-link-hover: #d1d5db;
    --color-footer-link-hover-bg: rgba(255, 255, 255, 0.06);
    --color-separator: #5a5a8a;
}

/* ===== Theme Transition ===== */
body,
.card,
footer {
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

.no-transition,
.no-transition *,
.no-transition *::before,
.no-transition *::after {
    transition: none !important;
}

html {
    font-size: 16px;
    -webkit-text-size-adjust: 100%;
}

body {
    font-family: system-ui, -apple-system, "Segoe UI", "PingFang SC", "Microsoft YaHei", sans-serif;
    line-height: 1.7;
    color: var(--color-text);
    background-color: var(--color-bg);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    padding: 0 24px;
}

/* ===== Main Container ===== */
main {
    width: 100%;
    max-width: 640px;
    padding: 48px 0;
    display: flex;
    flex-direction: column;
    gap: 32px;
}

/* ===== Header ===== */
header h1 {
    font-size: 1.75rem;
    font-weight: 700;
    color: var(--color-heading);
    margin-bottom: 8px;
}

.intro {
    color: var(--color-subtext);
    font-size: 2rem;
}

/* ===== Section Titles ===== */
h2 {
    font-size: 1rem;
    font-weight: 600;
    color: var(--color-subtext);
    margin-bottom: 12px;
    letter-spacing: 0.02em;
}

/* ===== Card List ===== */
.card-list {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.card {
    display: flex;
    flex-direction: column;
    gap: 4px;
    padding: 16px 20px;
    background: var(--color-card-bg);
    border: 1px solid var(--color-card-border);
    border-radius: 10px;
    text-decoration: none;
    transition: border-color 0.2s ease, box-shadow 0.2s ease, transform 0.15s ease, background-color 0.3s ease;
}

.card:hover {
    border-color: var(--color-card-hover-border);
    box-shadow: 0 2px 12px var(--color-card-hover-shadow);
    transform: translateY(-1px);
    background-color: var(--color-card-bg);
}

.card:active {
    transform: translateY(0);
    box-shadow: 0 1px 4px var(--color-card-active-shadow);
    background-color: var(--color-card-bg);
}

.card-title {
    font-size: 1rem;
    font-weight: 600;
    color: var(--color-card-title);
}

.card-desc {
    font-size: 0.85rem;
    color: var(--color-card-desc);
}

/* ===== Links ===== */
a {
    color: var(--color-link);
    text-decoration: none;
    position: relative;
    transition: color 0.2s ease, background-color 0.2s ease;
    border-radius: 4px;
    padding: 1px 0;
    margin: 0 2px;
}

a:hover {
    color: var(--color-link-hover);
    background-color: var(--color-link-hover-bg);
}

a:active {
    color: var(--color-link-active);
    background-color: var(--color-link-active-bg);
}

/* Card links override */
.card,
.card:hover,
.card:active {
    margin: 0;
    padding: 16px 20px;
}

/* ===== Footer ===== */
footer {
    margin-top: auto;
    padding-top: 24px;
    border-top: 1px solid var(--color-footer-border);
    font-size: 0.85rem;
    color: var(--color-footer-text);
}

footer p {
    margin-bottom: 6px;
}

.footer-links {
    display: flex;
    align-items: center;
    gap: 8px;
    flex-wrap: wrap;
}

.footer-links a {
    color: var(--color-footer-link);
    font-size: 0.85rem;
}

.footer-links a:hover {
    color: var(--color-footer-link-hover);
    background-color: var(--color-footer-link-hover-bg);
}

.separator {
    color: var(--color-separator);
}

/* ===== Theme Toggle ===== */
.theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
    z-index: 100;
    width: 40px;
    height: 40px;
    border: 1px solid var(--color-card-border);
    border-radius: 50%;
    background: var(--color-card-bg);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background-color 0.3s ease, border-color 0.3s ease, transform 0.15s ease;
    box-shadow: 0 1px 4px var(--color-card-active-shadow);
}

.theme-toggle:hover {
    border-color: var(--color-card-hover-border);
    box-shadow: 0 2px 8px var(--color-card-hover-shadow);
    transform: scale(1.05);
}

.theme-toggle:active {
    transform: scale(0.95);
}

.theme-toggle svg {
    width: 20px;
    height: 20px;
    fill: none;
    stroke: var(--color-subtext);
    stroke-width: 2;
    stroke-linecap: round;
    stroke-linejoin: round;
    transition: stroke 0.3s ease;
}

/* Icon visibility: default (system) */
.theme-toggle .icon-sun { display: none; }
.theme-toggle .icon-moon { display: none; }
.theme-toggle .icon-system { display: block; }

[data-theme="light"] .theme-toggle .icon-sun { display: block; }
[data-theme="light"] .theme-toggle .icon-moon { display: none; }
[data-theme="light"] .theme-toggle .icon-system { display: none; }

[data-theme="dark"] .theme-toggle .icon-sun { display: none; }
[data-theme="dark"] .theme-toggle .icon-moon { display: block; }
[data-theme="dark"] .theme-toggle .icon-system { display: none; }

/* ===== Responsive: Tablet ===== */
@media (max-width: 768px) {
    body {
        padding: 0 20px;
    }

    main {
        padding: 40px 0;
        gap: 28px;
    }

    header h1 {
        font-size: 1.5rem;
    }
}

/* ===== Responsive: Mobile ===== */
@media (max-width: 480px) {
    body {
        padding: 0 16px;
    }

    main {
        padding: 32px 0;
        gap: 24px;
    }

    header h1 {
        font-size: 1.35rem;
    }

    .card {
        padding: 14px 16px;
    }

    .card,
    .card:hover,
    .card:active {
        padding: 14px 16px;
    }

    .theme-toggle {
        top: 12px;
        right: 12px;
        width: 36px;
        height: 36px;
    }
}