The CSSStyleSheet.deleteRule() method removes a style rule from the current style sheet object.

API CSSOM CSSStyleSheet Method NeedsBrowserCompatibility NeedsMobileBrowserCompatibility Reference

CSSStyleSheet.insertRule() method inserts a new CSS rule into the current style sheet. (with some restrictions)

API CSSOM CSSStyleSheet Method NeedsBrowserCompatibility NeedsMarkupWork NeedsMobileBrowserCompatibility Reference