Preview is not accurate for the site branding block

AFAICT this is because some of Olivero's styles (aka the default theme aka system.theme:default in the screenshot) are not injected into the component's css and/or header_js, for its default_markup.

IOW: \Drupal\experience_builder\Plugin\ExperienceBuilder\ComponentSource\BlockComponent::getClientSideInfo() would need to change

return ['build' => $this->renderComponent([], $component->uuid())];

to something like

$build = $this->renderComponent([], $component->uuid());
// Attach the default theme's …
$build['#attached']['library'][] = …
return ['build' => $build];

Although arguably, this is a bug in \Drupal\system\Plugin\Block\SystemBrandingBlock::build() … but as far as that implementation is concerned, XB's rendering-in-isolation of this block is an unexpected use case.

But I think I see a possible heuristic for this:

$build = $this->renderComponent([], $component->uuid());
if (!empty(array_intersect($build['#cache']['tags'], ['config:system.site', 'config:system.theme'])) {
  // Attach the default theme's …
  $build['#attached']['library'][] = …
}
return ['build' => $build];

Something like that?