There has been a bit of discussion recently about the new CSS serving technology in Essential and child themes. So, to prove a point, I have created 'Essentials'. It is distributed with the theme itself: https://github.com/gjb2048/moodle-theme_essential/tree/master/essentials - and is a 'pure' starting point for theme developers to be in a position to clone and make child theme's of Essential.
Currently it requires the pre-release 18.104.22.168 of Essential in: https://github.com/gjb2048/moodle-theme_essential. This is because I found the drop-down menu text went white when hovered over. Odd I thought and it looks like a core oddity: https://github.com/moodle/moodle/blob/MOODLE_28_STABLE/theme/bootstrapbase/less/bootstrap/variables.less#L128-L129. So I had to adjust Essential's LESS and recompile. Therefore, TEST servers only, but then you'd be developing a theme on a test server anyway.
I have put instructions in the 'essentials/README.txt' file on how to install it. As I say in Essential's README.txt "The 'essentials' sub-folder is just a place to store and distribute the child theme. It will NOT be available until you install it.".
'Essentials' is provided 'AS IS' for theme developers, its there for you to learn from. It is not meant to be a full blown working theme with all the bugs removed. Where possible I have anticipated changes and added comments to help you.
If all goes well, then I'll back-port to the M2.7 version. Essential is due a release anyway, given the current list of changes.