The styleMap read-only property of the CSSStyleRule interface returns a StylePropertyMap object which provides access to the rule's property-value pairs.

A StylePropertyMap object.

The following example shows styleMap being used to modify a style using the StylePropertyMap.set() method.

const stylesheet = document.styleSheets[0];

Object.values(stylesheet.cssRules).forEach((block) => {
  if (block.selectorText === "button") {
    block.styleMap.set("--mainColor", "black");
  }
});