Theme/ThemeHandler.php
public function setCurrentTheme($identifier)
    {
        $theme = $this->getTheme($identifier);
        $this->container->get('session')->set('_admin_theme', $theme);
    }
public function setCurrentTheme($identifier)
    {
        $theme = $this->getTheme($identifier);
        $this->container->get('session')->set('_admin_theme', $theme);
    }
Sets the current theme

@param string $identifier [description]

@return null