getAllCssVariableNames = () => {
  return Array.from(document.styleSheets)
    .flatMap((sheet) => Array.from(sheet.cssRules))
    .flatMap((rule) => rule.style && Array.from(rule.style))
    .filter((key) => key && key.startsWith('--'))
}
Gets all variables name from current document stylesheets
@returns {Array} string