lib/Twig/Loader/Themes.php
public function useTheme($theme)
    {
        if (!in_array($theme, $this->fsloader->getNamespaces())) {
            throw new Twig_Error_Loader(sprintf('Theme "%s" not registered.', $theme));
        }

        $this->theme = $theme;
    }
public function useTheme($theme)
    {
        if (!in_array($theme, $this->fsloader->getNamespaces())) {
            throw new Twig_Error_Loader(sprintf('Theme "%s" not registered.', $theme));
        }

        $this->theme = $theme;
    }
Set loader theme.

@param $theme
@throws Twig_Error_Loader