I bought a premium theme, installed it, and the site crashed with a WSOD, so I figured changing the theme back to Bartik would probably solve it. Thus, after reading the very fresh Drush 8 documentation I ran the command (also tried other variations that didn't work):
drush config-set system.theme default bartik
Yet, when I did that I got the same error I had before I moved the Dev environment from Windows10 to Ubuntu, that very same error "the demon who haunted me" that says (paraphrase): "Please use a more functional environment for bootstrapping Drush"... I mean, even Ubuntu isn't? Come on...
System details: Ubuntu desktop 15.10, apache 2.4.12, php 5.6.11, mysql 5.6.27... full session: