Layering surfaces instead of picking one dark gray
The biggest structural mistake in dark mode design is treating the background as a single value. Interfaces with just one dark gray for all surfaces — navigation, cards, modals, tooltips — look flat and difficult to parse. The solution is elevation: assigning progressively lighter dark tones to progressively elevated surfaces, so the visual hierarchy that used to come from shadows and white backgrounds comes instead from lightness levels. A practical system: base layer at L:10-12% (the deepest background), navigation at L:13-15%, card surfaces at L:17-19%, modal overlay at L:21-23%, tooltip at L:25-27%. Each step is a 2-4% lightness increment — barely visible as a color swatch but clearly legible as depth when rendered in context.
