defineCSS function
        
CssStyleDeclaration
defineCSS(
    
- CssStyleDeclaration? currentCSS,
- CssStyleDeclaration? appendCSS, [
- dynamic defaultCSS
Defines a new CssStyleDeclaration merging currentCSS and appendCSS.
defaultCSS if currentCSS and appendCSS are null.
Implementation
CssStyleDeclaration defineCSS(
    CssStyleDeclaration? currentCSS, CssStyleDeclaration? appendCSS,
    [dynamic defaultCSS]) {
  if (currentCSS == null) {
    return appendCSS ?? asCssStyleDeclaration(defaultCSS);
  } else if (appendCSS == null) {
    return currentCSS;
  } else {
    return CssStyleDeclaration()
      ..cssText = '${currentCSS.cssText!} ; ${appendCSS.cssText!}';
  }
}