Restructure/rewrite Sass to resolve mixed declaration deprecation warnings

**Describe the bug**

Sass is changing the way it handles declarations mixed with nested rules to match the specification decision by the CSS working group:

https://sass-lang.com/documentation/breaking-changes/mixed-decls/

As explained in #5264, we currently have numerous warnings being thrown by Sass due to our uses of mixed declarations in the codebase. These will be suppressed by #5295, but the long-term solution is to rewrite the Sass itself to resolve these issues.

**To Reproduce**

Steps to reproduce the behavior:

1. Remove any warning/error suppression in our Sass build tasks
2. Run our Sass build tasks
3. Witness deprecation errors/warnings

**Expected behavior**

Our Sass should be rewritten to produce the same output that it currently does while simultaneously adhering to the new CSS specification for mixed declarations.

Remove the warning/error suppression from the Sass build tasks and run those tasks to inventory the mixed declaration warnings. Review the affected Sass across the codebase; done means the Sass follows the new mixed-declaration behavior, produces the same output, and builds without the deprecation warnings described in the issue.

Written by the indexing model from the issue text.