[25 September, 2017] Update to Bootstrap-4.0.0-beta

The new version from September 25, 2017 includes update from Bootstrap-4.0.0-alpha.6 to Bootstrap-4.0.0-beta. The alpha version still comes with the Olympus package and can be found in "Bootstrap alpha ( deprecated )" folder.

Migration to the actual version Bootstrap-4.0.0-beta was fulfilled according to the detailed instructions and recommendation of the Bootstrap team. Below you can find all required information on this matter:

https://medium.com/wdstack/upgrade-bootstrap-4-alpha-6-to-beta-ca582f15ee32
https://www.bootstrapdash.com/bootstrap-4-beta-whats-new/
https://wireddots.com/blog/upgrade-bootstrap-4-alpha-6-to-beta
https://getbootstrap.com/docs/4.0/layout/grid/#customizing-the-grid
http://getbootstrap.com/
https://getbootstrap.com/docs/4.0/getting-started/introduction/
As usual all changes (variables and their values) and fixes were amended in the separate files  Bootstrap\scss\_custom.scss and app\sass\theme-styles\_bootstrap-customization.scss ( styles of standard form elements, headings, paragraphs, lists etc). 
It should be pointed out, that upon Bootstrap-4.0.0-beta installation the files  app/Bootstrap/scss/bootstrap.scss, app/Bootstrap/scss/bootstrap-grid.scss, app/Bootstrap/scss/bootstrap-reboot.scss didn't have \Bootstrap\scss\_custom.scss file enabled by default. Thus, upon Bootstrap-4.0.0-beta installation, custom.scss file was enabled in the above mentioned files AFTER!!! Bootstrap\scss\_variables.scss file:
@import "variables";
@import "custom";
Changes in the HTML markup consisted of renaming classes and editing Bootstrap-Grid classes in accordance with the documentation on Alpha to Beta versions migration.