* Update `.nav-divider` mixin to simplify to a 1px line (one value
instead of two in parametric mixin)
* Rename and consolidate dropdown caret vars
* Add vars to Customizer
* Remove `@carousel-control-hover-color` since that's just the same as
the default control color and all we change is `opacity` (it's required
for specificity).
* Add vars to customizer
* Update customizer nav to include new section
* Remove the `li` from the var names
* Consolidate and simplify `text-shadow` vars