::selection

Drag-select text on almost any site and you get the same OS-blue highlight. It's one of the easiest places to plant a brand color - a single CSS rule, no markup change, no JavaScript.

::selection {
  background-color: #17c3fa;
  color: white;
}

The catch is that ::selection only honors a narrow set of properties: color, background-color, text-decoration, text-shadow, and the SVG fill / stroke pair. Anything else - background-image, padding, font-size, transitions - is silently ignored. The browser paints a flat overlay on top of the real text, it doesn't render a separate element you can fully style.

Treat it as a brand-color slot, not a full style hook, and check contrast on the way out. The new color sits over arbitrary content, so a soft accent that looks right on the hero can quietly fail WCAG against body copy further down the page.