FML/Stylesheet/Style.php
public function addStyleClass($styleClass)
    {
        $styleClass = (string)$styleClass;
        if (!in_array($styleClass, $this->styleClasses)) {
            array_push($this->styleClasses, $styleClass);
        }
        return $this;
    }
public function addStyleClass($styleClass)
    {
        $styleClass = (string)$styleClass;
        if (!in_array($styleClass, $this->styleClasses)) {
            array_push($this->styleClasses, $styleClass);
        }
        return $this;
    }
public function addStyleClass ( $ styleClass ) { $ styleClass = ( string ) $ styleClass ; if ( ! in_array ( $ styleClass , $ this -> styleClasses ) ) { array_push ( $ this -> styleClasses , $ styleClass ) ; } return $ this ; }
Add style class

@api
@param string $styleClass Style class
@return static
Add style class