Today, the method to activate fall-back has been changed. Old version of Legacy_AdminRenderSystem has used the 'theme_fromfile' value of XoopsConfig for this. But, I heard that many users activate 'theme_fromfile' on a routine basis. In their XOOPS Cube Legacy, Legacy_AdminRenderSystem always compiles templates of themes.
Therefore, Leagcy_AdminRenderSystem stopped using 'theme_fromfile'. And, it began using SiteConfig.
Admin-Theme developers should write the following value in their site_custom.ini.php.
When a user breaks the control panel with white-screen, he has to remove the theme directory, and have to write this value or clear templates_c.