registerRootDesign function
Registers global root design CSS in the browser document head.
Implementation
void registerRootDesign(RootDesign design) {
if (_registeredRootDesigns.contains(design.name)) return;
final cssText = design.cssText;
if (cssText.trim().isEmpty) return;
final head = web.document.querySelector('head');
if (head == null) return;
final element = web.document.createElement('style');
element.setAttribute('data-flint-root-design', design.name);
element.textContent = cssText;
head.appendChild(element);
_registeredRootDesigns.add(design.name);
}