https://www.w3.org/TR/css-variables/

2020
https://increment.com/frontend/a-users-guide-to-css-variables/
2018
https://css-tricks.com/dry-state-switching-with-css-variables-fallbacks-and-invalid-values/
https://css-tricks.com/dry-switching-with-css-variables-the-difference-of-one-declaration/
https://css-tricks.com/updating-a-css-variable-with-javascript/
https://css-tricks.com/tag/custom-properties/
https://www.sitepoint.com/practical-guide-css-variables-custom-properties/
CSS Variables Tutorial (CSS Custom Properties)
https://www.youtube.com/watch?v=sQUB039MG0I
CSS Variable Secrets
https://www.youtube.com/watch?v=UQRSaG1hQ20
CodingTech - The Strategy Guide to CSS Custom Properties
https://www.youtube.com/watch?v=U9UU_fgpmO8
https://www.smashingmagazine.com/2018/05/css-custom-properties-strategy-guide/

2017
https://codeburst.io/a-quick-guide-to-css-variables-6f7f5c7465dd
    https://habr.com/company/poiskvps/blog/422487/
https://www.smashingmagazine.com/2017/04/start-using-css-custom-properties/