update = (css, sheet, append, cssToReplace) => {
    cssToReplace
        ? (sheet.data = sheet.data.replace(cssToReplace, css))
        : sheet.data.indexOf(css) === -1 &&
          (sheet.data = append ? css + sheet.data : sheet.data + css);
}
Updates the target and keeps a local cache
@param {String} css
@param {Object} sheet
@param {Boolean} append
@param {?String} cssToReplace