Update: After removing the namespace, the changes were displayed.
This is the solution:
Code is placed in : theme/knowbase/classes/output/core_search_renderer.php . Theme knowbase inherits from theme boost.
defined('MOODLE_INTERNAL') || die;
class theme_knowbase_core_search_renderer extends \core_search\output\renderer {
public function render_results($results, $page, $totalcount, $url, $cat = null) {
}
}