Modern Responsive Portfolio Site

I need a sleek personal portfolio built entirely with semantic HTML5 and well-structured CSS3. The look must feel premium: dark theme, smooth gradient backgrounds, generous white-space (well, dark-space), and crisp sans-serif typography that scales gracefully from mobile to large desktop.

Content sections are already defined—hero, about, skills (emphasis on my HTML/CSS strengths), project cards, services, contact, and a footer with social links—so your task is to translate these into a polished, cohesive layout. I want subtle hover transitions, refined spacing, and clean code that scores high on performance tools without leaning on JavaScript unless a tiny interaction truly demands it.

Deliverables
• Fully responsive HTML/CSS files for every section listed above
• Reusable CSS variables for colours, spacing, and gradients to keep maintenance simple
• Commented source code explaining key structural or styling decisions
• A quick read-me describing how to add or swap portfolio items

I’ll review on modern browsers and multiple breakpoints; if the design loads fast, adapts smoothly, and reflects the minimal yet high-end aesthetic I described, we’re done.