Ultra Photorealistic Gym Mirror Selfie Prompt (Billie Eilish Style)

You are an expert web designer. Build a stunning, production-ready page with
a bold hero section, smooth CSS transitions and a carefully tuned dark palette.
Use a neutral background, generous rounded corners and one electric accent color.
Structure the layout with a sticky header, feature grid, social proof and a final
call to action. Keep typography tight, spacing consistent and motion subtle.