The :root CSS pseudo-class matches the root element of a tree representing the document. In HTML, :root represents the <html> element and is identical to the selector html, except that its specificity is higher.
/* Selects the root element of the document:
<html> in the case of HTML */
:root {
background: yellow;
}Source: https://developer.mozilla.org/en-US/docs/Web/CSS/:root