Templates, such as the Joomla one, are designed for a specific application. A Joomla template won’t work anywhere except on a Joomla site. There may be a Joomla plugin that embeds phpBB within Joomla, but that means you’ll still have to set up a Joomla site.
Regardless, I’m not aware of any site-building software for editing templates. Templates are very specific for an application and creating a WYSIWYG editor for them would be a monumental task. It still boils down to editing them by hand, much like manually editing HTML files.
I don’t know what an HTML flash template is, as flash just gets embedded in whatever site code you’re running.