To copy the layout to your own template folder, this is the best approach to solve this :
I am not sure, why you can't access to the plugin settings.
Create a template extension (sitepackage):
copy the file
typo3conf/ext/sitepackage/Resources/Private/Fluid/Content/Layouts/HeaderContentFooter.html
add this to TypoScript
:
plugin.tx_sitepackage {
view {
templateRootPaths.0 = EXT:sitepackage/Resources/Private/Templates/
partialRootPaths.0 = EXT:sitepackage/Resources/Private/Partials/
layoutRootPaths.0 = EXT:sitepackage/Resources/Private/Layouts/
}
}
add something like this to HeaderContentFooter.html
{namespace css=Vendor\Sitepackage\ViewHelpers}
and to render the CSS class
{css:getclass()}
Create a Viewhelper in
sitepackage/Classes/ViewHelpers/getclass.php
with something like this inside:
namespace Vendor\Sitepackage\viewhelper ;
class getclassViewHelper extends \TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper {
public function render() {
$objectManager = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Extbase\\Object\\ObjectManager');
$configurationManager = $objectManager->get('TYPO3\\CMS\\Extbase\\Configuration\\ConfigurationManagerInterface');
$settings = $configurationManager->getConfiguration(\TYPO3\CMS\Extbase\Configuration\ConfigurationManagerInterface::CONFIGURATION_TYPE_FULL_TYPOSCRIPT );
return $settings['FlexFormfieldName'] ;
}
}
Not tested Completely but should do so