:root {
    --header-height: 72px;
    --blur-intensity-high: 15px;
    --blur-intensity-medium: 10px;
    --blur-intensity-low: 5px;
    --saturate-intensity: 130%;

    /* --- Default Theme: Light Cyan --- */
    --bg-color: #e0f7fa; 
    --glass-bg: rgba(255, 255, 255, 0.5); 
    --glass-border: rgba(224, 247, 250, 0.7);
    --outer-section-card-bg: rgba(255, 255, 255, 0.3);
    --outer-section-card-border: rgba(224, 247, 250, 0.5);
    --text-primary: #0c4a6e; 
    --text-secondary: #0e7490;
    --text-accent: #0891b2; 
    --text-accent-hover: #06b6d4;
    --icon-color-primary: var(--text-accent);
    --section-title: #0c4a6e;
    --section-subtitle: #0e7490;
    --button-bg: rgba(8, 145, 178, 0.15);
    --button-border: rgba(8, 145, 178, 0.3);
    --button-text: var(--text-accent);
    --button-hover-bg: rgba(8, 145, 178, 0.25);
    --scrollbar-track: rgba(0, 0, 0, 0.05);
    --scrollbar-thumb: rgba(8, 145, 178, 0.5);
    --hero-content-bg: rgba(255, 255, 255, 0.25);
    --header-bg: rgba(207, 250, 254, 0.35); 
    --mobile-menu-bg: rgba(207, 250, 254, 0.9);
    --modal-bg: rgba(224, 247, 250, 0.5);
    --settings-button-active-bg: var(--text-accent);
    --settings-button-active-text: #ffffff;
}

/* --- Gray Theme (Supports Auto Dark Mode) --- */
[data-theme="gray"][data-mode="light"] {
    --bg-color: #f3f4f6;
    --glass-bg: rgba(255, 255, 255, 0.55);
    --glass-border: rgba(229, 231, 235, 0.75);
    --outer-section-card-bg: rgba(255, 255, 255, 0.35);
    --outer-section-card-border: rgba(229, 231, 235, 0.6);
    --text-primary: #1f2937;
    --text-secondary: #4b5563;
    --text-accent: #b45309;
    --text-accent-hover: #d97706;
    --button-bg: rgba(180, 83, 9, 0.15);
    --button-border: rgba(180, 83, 9, 0.3);
    --scrollbar-thumb: rgba(180, 83, 9, 0.5);
    --header-bg: rgba(243, 244, 246, 0.35);
    --mobile-menu-bg: rgba(243, 244, 246, 0.9);
    --modal-bg: rgba(229, 231, 235, 0.55);
}
[data-theme="gray"][data-mode="dark"] {
    --bg-color: #374151; 
    --glass-bg: rgba(75, 85, 99, 0.45); 
    --glass-border: rgba(107, 114, 128, 0.55);
    --outer-section-card-bg: rgba(75, 85, 99, 0.3);
    --outer-section-card-border: rgba(107, 114, 128, 0.4);
    --text-primary: #f3f4f6; 
    --text-secondary: #d1d5db;
    --text-accent: #f59e0b; 
    --text-accent-hover: #fcd34d;
    --button-bg: rgba(245, 158, 11, 0.15);
    --button-border: rgba(245, 158, 11, 0.3);
    --scrollbar-thumb: rgba(245, 158, 11, 0.5);
    --hero-content-bg: rgba(55, 65, 81, 0.4);
    --header-bg: rgba(55, 65, 81, 0.35);
    --mobile-menu-bg: rgba(55, 65, 81, 0.9);
    --modal-bg: rgba(75, 85, 99, 0.6);
    --settings-button-active-text: #422006; 
}

/* --- Purple Theme (Always Light Mode) --- */
[data-theme="purple"] {
    --bg-color: #f5f3ff;
    --glass-bg: rgba(255, 255, 255, 0.5);
    --glass-border: rgba(237, 233, 254, 0.8);
    --outer-section-card-bg: rgba(255, 255, 255, 0.3);
    --outer-section-card-border: rgba(237, 233, 254, 0.6);
    --text-primary: #4c1d95;
    --text-secondary: #6d28d9;
    --text-accent: #8b5cf6;
    --text-accent-hover: #a78bfa;
    --button-bg: rgba(139, 92, 246, 0.15);
    --button-border: rgba(139, 92, 246, 0.3);
    --scrollbar-thumb: rgba(139, 92, 246, 0.5);
    --header-bg: rgba(245, 243, 255, 0.35);
    --mobile-menu-bg: rgba(245, 243, 255, 0.9);
    --modal-bg: rgba(237, 233, 254, 0.55);
    --settings-button-active-text: #ffffff;
}

/* --- Orange Theme (Always Light Mode) --- */
[data-theme="orange"] {
    --bg-color: #fff7ed;
    --glass-bg: rgba(255, 255, 255, 0.5);
    --glass-border: rgba(255, 237, 213, 0.8);
    --outer-section-card-bg: rgba(255, 255, 255, 0.3);
    --outer-section-card-border: rgba(255, 237, 213, 0.6);
    --text-primary: #7c2d12;
    --text-secondary: #9a3412;
    --text-accent: #f97316;
    --text-accent-hover: #fb923c;
    --button-bg: rgba(249, 115, 22, 0.15);
    --button-border: rgba(249, 115, 22, 0.3);
    --scrollbar-thumb: rgba(249, 115, 22, 0.5);
    --header-bg: rgba(255, 247, 237, 0.35);
    --mobile-menu-bg: rgba(255, 247, 237, 0.9);
    --modal-bg: rgba(255, 237, 213, 0.55);
    --settings-button-active-text: #ffffff;
}

/* Default Dark Mode (applied to default/cyan theme in dark mode) */
[data-mode="dark"]:not([data-theme="gray"]):not([data-theme="purple"]):not([data-theme="orange"]) {
    --bg-color: #0f172a; 
    --glass-bg: rgba(30, 41, 59, 0.5);
    --glass-border: rgba(51, 65, 85, 0.6);
    --outer-section-card-bg: rgba(30, 41, 59, 0.35);
    --outer-section-card-border: rgba(51, 65, 85, 0.45);
    --text-primary: #e2e8f0; 
    --text-secondary: #94a3b8;
    --text-accent: #818cf8; 
    --text-accent-hover: #a5b4fc;
    --button-bg: rgba(129, 140, 248, 0.15);
    --button-border: rgba(129, 140, 248, 0.3);
    --scrollbar-thumb: rgba(129, 140, 248, 0.5);
    --hero-content-bg: rgba(15, 23, 42, 0.5);
    --header-bg: rgba(15, 23, 42, 0.4);
    --mobile-menu-bg: rgba(15, 23, 42, 0.9);
    --modal-bg: rgba(30, 41, 59, 0.65);
    --settings-button-active-text: #e0e7ff; 
}

body {
    font-family: 'Inter', sans-serif;
    scroll-behavior: smooth;
    background-color: var(--bg-color);
    min-height: 100vh;
    padding-top: var(--header-height);
    color: var(--text-primary);
    transition: background-color 0.4s ease-in-out;
}

::-webkit-scrollbar { width: 8px; }
::-webkit-scrollbar-track { background: var(--scrollbar-track); }
::-webkit-scrollbar-thumb { background: var(--scrollbar-thumb); border-radius: 4px; }
::-webkit-scrollbar-thumb:hover { background: color-mix(in srgb, var(--scrollbar-thumb) 70%, var(--text-primary) 30%); }

.glass-effect {
    backdrop-filter: blur(var(--blur-intensity-medium)) saturate(var(--saturate-intensity));
    -webkit-backdrop-filter: blur(var(--blur-intensity-medium)) saturate(var(--saturate-intensity));
    border: 1px solid var(--glass-border);
}
.glass-header {
    background-color: var(--header-bg);
    backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity));
    -webkit-backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity));
    border-bottom: 1px solid var(--glass-border);
}
.outer-section-card {
    background-color: var(--outer-section-card-bg);
    backdrop-filter: blur(var(--blur-intensity-low)) saturate(var(--saturate-intensity));
    -webkit-backdrop-filter: blur(var(--blur-intensity-low)) saturate(var(--saturate-intensity));
    border: 1px solid var(--outer-section-card-border);
    transition: transform 0.3s ease-out, box-shadow 0.3s ease-out, background-color 0.3s, opacity 0.5s;
}
.outer-section-card:hover { box-shadow: 0 10px 25px rgba(0, 0, 0, 0.08); }
.glass-card {
    background-color: var(--glass-bg);
    transition: transform 0.3s ease-out, box-shadow 0.3s ease-out, background-color 0.3s, opacity 0.5s;
    color: var(--text-secondary);
}
.glass-card h3, .glass-card h4 { color: var(--text-primary); }
.glass-card:hover {
    transform: translateY(-5px) scale(1.01);
    box-shadow: 0 12px 24px rgba(0, 0, 0, 0.1), 0 0 0 1px color-mix(in srgb, var(--glass-border) 50%, var(--text-primary) 8%);
    background-color: color-mix(in srgb, var(--glass-bg) 90%, var(--text-primary) 3%);
}

.text-primary-on-glass { color: var(--text-primary); }
.text-secondary-on-glass { color: var(--text-secondary); }
.text-accent-on-glass { color: var(--text-accent); }
.text-accent-on-glass:hover { color: var(--text-accent-hover); }

.nav-link-glass { color: var(--text-secondary); }
.nav-link-glass.active {
    color: var(--text-primary);
    font-weight: 600;
    background-color: color-mix(in srgb, var(--header-bg) 75%, var(--text-primary) 15%);
}
.nav-link-glass:hover {
     background-color: color-mix(in srgb, var(--header-bg) 90%, var(--text-primary) 8%);
}

.hero-content-bg {
    background-color: var(--hero-content-bg);
    backdrop-filter: blur(var(--blur-intensity-low)) saturate(var(--saturate-intensity));
    -webkit-backdrop-filter: blur(var(--blur-intensity-low)) saturate(var(--saturate-intensity));
    border: 1px solid var(--glass-border);
}
.section-title { color: var(--section-title); text-shadow: 0 1px 2px rgba(0,0,0,0.05); }
.section-subtitle { color: var(--section-subtitle); text-shadow: 0 1px 1px rgba(0,0,0,0.03); }
.material-icons, .material-icons-outlined { vertical-align: middle; }

.glass-button {
    background-color: var(--button-bg);
    backdrop-filter: blur(var(--blur-intensity-low));
    -webkit-backdrop-filter: blur(var(--blur-intensity-low));
    border: 1px solid var(--button-border);
    color: var(--button-text);
    transition: background-color 0.3s, transform 0.3s, border-color 0.3s;
}
.glass-button:hover {
    background-color: var(--button-hover-bg);
    border-color: color-mix(in srgb, var(--button-border) 70%, var(--text-accent) 30%);
    transform: scale(1.03);
}

.ebook-card-icon {
    font-size: 2.75rem; margin-bottom: 0.65rem; color: var(--icon-color-primary);
}

.collapsible-header {
    cursor: pointer; display: flex; justify-content: center; align-items: center;
    padding: 16px 0; transition: background-color 0.3s; position: relative;
    border-radius: 1rem; 
}
.collapsible-icon {
    transition: transform 0.3s ease-out; font-size: 28px; margin-left: 12px;
    color: var(--text-accent);
}
.collapsible-content {
    max-height: 0; overflow: hidden;
    transition: max-height 0.4s cubic-bezier(0.25, 0.1, 0.25, 1), 
                padding-top 0.4s cubic-bezier(0.25, 0.1, 0.25, 1), 
                padding-bottom 0.4s cubic-bezier(0.25, 0.1, 0.25, 1), 
                opacity 0.3s 0.1s ease-out;
    padding-top: 0; padding-bottom: 0; opacity: 0;
}
.collapsible-content.expanded {
    max-height: 6000px; 
    padding-top: 1rem; padding-bottom: 2rem; opacity: 1;
}
section.collapsible-section.collapsed .collapsible-icon { transform: rotate(-90deg); }
.outer-section-card > .collapsible-section { padding: 0 !important; margin: 0 !important; }

.settings-modal-overlay {
    position: fixed; top: 0; left: 0; width: 100%; height: 100%;
    background-color: rgba(0,0,0,0.15); 
    backdrop-filter: blur(var(--blur-intensity-low)); -webkit-backdrop-filter: blur(var(--blur-intensity-low));
    z-index: 1000; display: flex; justify-content: center; align-items: center;
    opacity: 0; visibility: hidden; transition: opacity 0.3s ease, visibility 0s 0.3s;
}
.settings-modal-overlay.active { opacity: 1; visibility: visible; transition-delay: 0s; }
.settings-modal {
    background-color: var(--modal-bg); 
    backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity)); 
    -webkit-backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity));
    border: 1px solid var(--glass-border); border-radius: 16px; padding: 24px;
    box-shadow: 0 10px 30px rgba(0,0,0,0.15); color: var(--text-primary);
    width: 90%; max-width: 360px; position: relative;
    transform: translateY(-20px) scale(0.95); opacity: 0;
    transition: transform 0.3s cubic-bezier(0.25, 0.1, 0.25, 1), opacity 0.3s ease;
}
.settings-modal-overlay.active .settings-modal { transform: translateY(0) scale(1); opacity: 1;}
.settings-modal h3 { margin-top: 0; margin-bottom: 20px; font-size: 1.15rem; font-weight: 600; text-align: center;}
.settings-modal h4 { margin-top: 16px; margin-bottom: 8px; font-size: 0.95rem; font-weight: 500;}
.settings-modal .theme-buttons { display: flex; flex-wrap: wrap; gap: 8px; }
.settings-modal .theme-buttons button {
    padding: 8px 10px; border-radius: 8px; border: 1px solid var(--button-border);
    background-color: var(--button-bg); color: var(--text-secondary); cursor: pointer;
    transition: background-color 0.2s, color 0.2s, border-color 0.2s; font-size: 0.8rem; flex-grow: 1;
}
.settings-modal .theme-buttons button.active {
    background-color: var(--settings-button-active-bg);
    color: var(--settings-button-active-text);
    font-weight: 600;
    border-color: var(--settings-button-active-bg);
}
.settings-modal .theme-buttons button:hover:not(.active) {
    background-color: color-mix(in srgb, var(--button-bg) 70%, var(--text-accent) 30%);
    color: var(--text-primary);
    border-color: color-mix(in srgb, var(--button-border) 70%, var(--text-accent) 30%);
}
.settings-modal-close {
    position: absolute; top: 12px; right: 12px; background: none; border: none;
    color: var(--text-secondary); cursor: pointer; padding: 4px; border-radius: 50%;
}
.settings-modal-close:hover { color: var(--text-primary); background-color: rgba(0,0,0,0.1); }
[data-mode="light"] .settings-modal-close:hover { background-color: rgba(255,255,255,0.1); }

.reveal {
    opacity: 0;
    transform: translateY(15px);
    transition: opacity 0.5s ease-out, transform 0.5s ease-out;
}
.reveal.visible { opacity: 1; transform: translateY(0); }

.hamburger-button {
    width: 24px; height: 20px; position: relative; cursor: pointer;
    display: flex; flex-direction: column; justify-content: space-between;
}
.hamburger-button span {
    display: block; width: 100%; height: 2px;
    background-color: var(--text-primary); border-radius: 1px;
    transition: transform 0.3s cubic-bezier(0.68, -0.55, 0.27, 1.55), opacity 0.2s ease-in-out;
}
.hamburger-button.active span:nth-child(1) { transform: translateY(9px) rotate(45deg); }
.hamburger-button.active span:nth-child(2) { opacity: 0; transform: scaleX(0); }
.hamburger-button.active span:nth-child(3) { transform: translateY(-9px) rotate(-45deg); }

#mobile-menu {
    background-color: var(--mobile-menu-bg);
    backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity));
    -webkit-backdrop-filter: blur(var(--blur-intensity-high)) saturate(var(--saturate-intensity));
    transform: translateY(-110%);
    transition: transform 0.4s cubic-bezier(0.23, 1, 0.32, 1);
    position: fixed; top: var(--header-height); left: 0; right: 0;
    z-index: 40; border-top: 1px solid var(--glass-border);
}
#mobile-menu.active { transform: translateY(0); }