Hi dear,
\components\com_osproperty\helpers\helper.php
around line 900
these code must be changed if you have your own css for frontend_style.css
i made changes to red
// if (file_exists(JPATH_ROOT . $current_template . 'html/com_osproperty/css/frontend_style.css')) {
if (file_exists(JPATH_ROOT .'/templates/'. $current_template . '/html/com_osproperty/css/frontend_style.css')) {
//$document->addStyleSheet($rootUrl .'/'. $current_template.'/html/com_osproperty/css/frontend_style.css');
$document->addStyleSheet($rootUrl .'/templates/'. $current_template.'/html/com_osproperty/css/frontend_style.css');