Directly changing a theme causes a "Functional environment bootstrapping" error 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:
I wasn't inside the sites' folder! I didn't notice that for some reason - Here is a possible explanation why, that might be nice for you to read:

In the similar time period, I also ran a few general Drush commands inside the root folder just to check if Drush itself is working because of Environment Variable (Path) value setting mistakes I made earlier and tried to fix; I could run drush in each session only after manual var export, but I than went to home/.profile, and added export PATH="~/.composer/vendor/bin:$PATH" in the end of file and it fixed it.