Migrate sass @imports and global built-ins

[chrstinalin]

Fixes #15367

Adapt sass files to address deprecation warnings

Ran sass-migrator for:

• @import and global built-in functions
• Color Functions -- Nothing to migrate was found.
• Slash as Division

Manual changes for:

• Mixed Declarations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.13%. Comparing base (3d89a9b) to head (6b924d4).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14253   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files         267      267           
  Lines       10664    10664           
  Branches     3292     3292           
=======================================
  Hits        10465    10465           
  Misses        186      186           
  Partials       13       13           

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