header {
    position: fixed;
    top: 0;
    width: 100%;
    background: linear-gradient(90deg, var(--header-gradient-1), var(--header-gradient-2));
    backdrop-filter: blur(10px);
    height: var(--header-height);
    z-index: var(--z-header);
    border-bottom: 1px solid var(--border-color);
    left: 0;
    right: 0;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
    transition: var(--theme-transition), transform 0.7s var(--transition-timing), box-shadow 0.7s var(--transition-timing);
    transform: var(--gpu-accelerated);
    padding-left: var(--safe-area-inset-left);
    padding-right: var(--safe-area-inset-right);
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: var(--content-width-lg);
    margin: 0 auto;
    padding: 0 var(--container-padding);
    height: 100%;
}

.logo-container {
    display: flex;
    align-items: center;
    gap: var(--space-sm);
}

.logo-text {
    font-size: var(--font-size-xl);
    font-weight: var(--font-weight-bold);
    text-align: center;
    line-height: var(--line-height-tight);
}

.logo-text span:nth-child(1) { color: var(--primary-gradient-1); }
.logo-text span:nth-child(2) { color: var(--primary-gradient-2); }
.logo-text span:nth-child(3) { color: var(--primary-gradient-3); }
.logo-text span:nth-child(4) { color: var(--primary-gradient-4); }

.nav-menu {
    display: flex;
    gap: var(--space-lg);
    align-items: center;
    padding-top: var(--space-md);
}

.nav-menu a {
    color: var(--text);
    text-decoration: none;
    font-size: var(--font-size-md);
    font-weight: var(--font-weight-medium);
    transition: all var(--transition-speed) var(--transition-timing);
    padding: var(--space-xs) var(--space-sm);
    border-radius: var(--radius-md);
    position: relative;
    background-color: rgba(var(--bg-hover-nav));
    letter-spacing: 0.3px;
}

.nav-menu a::after {
    content: '';
    position: absolute;
    width: 0;
    height: 2px;
    bottom: 0;
    left: 50%;
    background-color: var(--primary);
    transition: all var(--transition-speed) var(--transition-timing);
    transform: translateX(-50%);
}

.nav-menu a:hover {
    color: var(--primary);
    background-color: var(--bg-card);
    transform: translateY(-1px);
}

.nav-menu a:hover::after {
    width: calc(100% - var(--space-xl));
}

.theme-toggle {
    background: none;
    border: none;
    color: var(--text);
    cursor: pointer;
    padding: var(--space-sm);
    display: flex;
    align-items: center;
    justify-content: center;
    transition: var(--theme-transition);
    border-radius: var(--radius-md);
    position: relative;
    width: calc(var(--space-xl) * 1.25);
    height: calc(var(--space-xl) * 1.25);
    margin-left: var(--space-sm);
}

.theme-toggle:hover {
    color: var(--primary);
    background-color: var(--bg-card);
}

.theme-toggle:focus-visible {
    outline: 2px solid var(--primary);
    outline-offset: 2px;
    border-radius: var(--radius-md);
}

.theme-toggle i {
    font-size: var(--font-size-lg);
    position: absolute;
    transition: opacity var(--transition-speed) var(--transition-timing),
                transform var(--transition-speed) var(--transition-timing);
    transform: var(--gpu-accelerated);
}

/* Light Theme Icon States */
[data-theme="light"] .theme-toggle .fa-moon {
    opacity: 1;
    transform: rotate(0) var(--gpu-accelerated);
}

[data-theme="light"] .theme-toggle .fa-sun {
    opacity: 0;
    transform: rotate(-90deg) var(--gpu-accelerated);
}

/* Dark Theme Icon States */
[data-theme="dark"] .theme-toggle .fa-moon {
    opacity: 0;
    transform: rotate(90deg) var(--gpu-accelerated);
}

[data-theme="dark"] .theme-toggle .fa-sun {
    opacity: 1;
    transform: rotate(0) var(--gpu-accelerated);
}

/* Hover Animation */
.theme-toggle:hover i {
    transform: rotate(15deg) var(--gpu-accelerated);
}

.mobile-menu-toggle {
    display: none;
    background: none;
    border: none;
    color: var(--text);
    cursor: pointer;
    padding: var(--space-sm);
}