Almost no great site uses #000000 or #ffffff. The small offsets are where "premium" actually lives.
Here is a test you can run in ten seconds. Open a site you think looks expensive, inspect the background, and read the computed color. I have done this hundreds of times now, and the answer is almost never #ffffff. It is almost never #000000 either. The pure values look like defaults because they are defaults.
Yes, 82% of the sites I measured are light-themed — but "light" lives in a warmed near-white range, roughly #faf8f4 to #fdfdfb, usually with a single tinted surface for cards. Pure white is reserved, if it appears at all, for a surface that needs to lift off the base. A background one or two percent off pure white reads calmer and more considered, and most people cannot tell you why. That "why" is the offset.
The dark sites follow the same logic in reverse. Measured dark backgrounds cluster around #0a0a0b to #141416 — near-black, never true black. Pure black with light text vibrates and strains the eye, and it removes your ability to use true black for depth. The good dark UIs keep black in reserve.
The same restraint applies to text. Body copy on light is rarely pure black (#333 to #555 dominates); body copy on dark is rarely pure white. Hierarchy comes from luminance steps — a near-black or near-white for headings, a dimmer tone for body, a muted tone for labels. Three tiers, none of them at the extreme.
These offsets are individually invisible and collectively decisive. They are a large share of what people mean when they say a site "feels designed." And they are trivial to get right once you stop typing the default values out of habit. If you build with AI tools, specify them — the model will happily hand you #ffffff and #000000 unless you tell it not to. I collected the working ranges in the palette guide and, for dark specifically, in the dark mode guide.