Update "Theming" documentation to include info on using CSS vars

Created by: willthemoor

Loving BS4. Needed to make a few tweaks to the default colors so I installed the SCSS via NPM, created my own _variables.scss doc, updated links, blah blah blah. All the while I'm thinking, "shouldn't BS4 use CSS vars for this?". Lo! I inspected an element for some other reason and saw a beautiful set of --vars on the :root element! 😃 🏕

I'm already invested in the custom SCSS route (and need to adjust a few other variables not accounted for in _root.scss) but I'm very glad to learn of this feature. If https://getbootstrap.com/docs/4.0/getting-started/theming/ had info on a) their availability and b) how to use them, I might have just sorted it all out with --vars and saved a bunch of time.

This is a really fantastic feature update to Bootstrap; this issue is me hoping it gets the documentation attention I reckon it deserves. TBH, my CSS Variables fu is weak (been hooked on pre-processors for so long) or I'd offer up a starter PR. Still, down to help get it going if someone wants to point my inner Energizer Bunny in the right direction.