* Rewrite custom form check backgrounds
Previously, this was all just a background-color and background-image. When we restored the gradients though, we had two background-images competing on the same element, causing rendering glitches. This refactors that code, creating a mixin to simplify things, so we can we easily use two background-images (SVG icon and gradient) when -gradients is set to true.
Fixes#24598
* restore default vars
* Revamp custom check and radio backgrounds
Instead of applying multiple background-image's to the same element, we're adding a new ::before pseudo-element to layer the background-images. Gradients go on the .custom-control-indicator while their icons go on the ::before element. This allows us to shave some bytes from when we compile and we previously needed to redeclare the background-image for the icon if you changed the gradient.
* remove now unused mixin
* mention change in migration docs
* Change breakpoint max- calculation to fractional values
* Update docs to reflect fractional max-width breakpoint values
* Add fractional max-width to offcanvas example
* Add documentation and SCSS comment for fractional viewport support
* Add .modal-dialog-centered for optional vertically cenetered modal
Fixes#23638
* adds modal-dialog-centered class to docs and removes margin to avoid generating a vertical scrolling
* mention limitations
* fix aria attr
* Add `width: 100%` to the .modal-content for the centered version. Adding it here to avoid adding another selector by limiting it to the centered modal modifier.
These were using `z-index: 2` to "Place active items above their
siblings for proper border styling". However, using `z-index: 1` is
sufficient for accomplishing that goal.
In input-group, there were also three `z-index: 3` rules for the
hover/focus/active states. I reduced these to `z-index: 2` since they
just needed to be "one more than normal" (i.e. one more than what is now
`z-index: 1` after my changes).
These changes can be verified by viewing the documentation pages for
Button group, Input group, List group, and Pagination before and after
this commit and observing that the active elements are still "above"
their siblings, so their borders look correct.
