.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 clamp(1rem, 5vw, 1.5rem);
}

.header {
    padding: clamp(1rem, 4vw, 2rem) 0;
    text-align: center;
    background: var(--card-bg-soft);
    border-bottom: 1px solid rgba(148, 163, 184, 0.14);
    backdrop-filter: blur(12px);
}

.header-row {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: clamp(0.5rem, 2vw, 1rem);
    flex-wrap: nowrap;
}

.logo {
    font-weight: 700;
    font-size: clamp(1rem, 4vw, 1.25rem);
    color: var(--text-color);
    white-space: nowrap;
    flex-shrink: 0;
}

.nav a {
    margin-left: 1rem;
    color: var(--text-muted);
    font-weight: 600;
}

.nav a:hover {
    color: var(--text-color);
}

.theme-toggle {
    margin-left: clamp(0.5rem, 2vw, 1rem);
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    flex-shrink: 0;
    min-height: 44px;
    min-width: 44px;
}

.theme-toggle svg {
    width: 16px;
    height: 16px;
}

.tagline {
    color: var(--text-muted);
    margin-top: 0.5rem;
    font-size: clamp(0.8rem, 2vw, 0.9rem);
}

.problem-heading {
    font-size: clamp(1rem, 3vw, 1.1rem);
    font-weight: 700;
}

.main-content {
    padding: clamp(1rem, 5vw, 2rem) 0;
}

/* Responsive grid: 3 cols desktop, 2 cols tablet, 1 col mobile */
.grid {
    display: grid;
    gap: clamp(1rem, 3vw, 1.5rem);
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
}

@media (max-width: 1024px) {
    .grid {
        grid-template-columns: repeat(2, 1fr);
    }
}

@media (max-width: 640px) {
    .container {
        padding: 0 clamp(1rem, 4vw, 1rem);
    }
    
    .grid {
        grid-template-columns: 1fr;
    }
    
    .header-row {
        gap: 0.5rem;
        flex-wrap: wrap;
        justify-content: center;
    }
    
    .logo {
        font-size: clamp(0.9rem, 3.5vw, 1rem);
    }
    
    .problem-heading {
        min-width: 100%;
        order: 3;
        margin-top: 0.5rem;
        margin-bottom: 0.5rem;
    }
}

@media (max-width: 374px) {
    .problem-heading {
        font-size: clamp(0.85rem, 3vw, 0.95rem);
    }
}

footer {
    padding: 2rem 0;
    text-align: center;
    color: var(--text-muted);
    font-size: clamp(0.75rem, 2vw, 0.875rem);
}