highlight-color.js

// https://flatuicolors.com/palette/ca
const colors = ["#ff9ff3", "#feca57", "#ff6b6b", "#48dbfb", "#1dd1a1"];
window.addEventListener("mousedown", (e) => {
  const color = colors.shift();
  document.documentElement.style.setProperty("--highlight-color", color);
  colors.push(color);
});

main.css

:root {
  --highlight-color: null;
}

::selection {
  background: var(--highlight-color);
}