[Tech-Debt] FAM Sass Warnings

[ianliuwk1019]

Describe the task
When starting up FAM frontend locally and accessing pages, on the running console outputs many Sass deprecation warnings:

• Use of @import:

Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import

  ╷
1 │ @import "mixins/banner";
  │         ^^^^^^^^^^^^^^^

• Global build-in function:

Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.      
Use color.mix instead.

More info and automated migrator: https://sass-lang.com/d/import

    ╷
342 │ $light-bg-subtle:         mix($gray-100, $white) !default;
    │                           ^^^^^^^^^^^^^^^^^^^^^^
    ╵

• Color function deprecation:

Deprecation Warning [color-functions]: green() is deprecated. Suggestion:

color.channel($color, "green", $space: rgb)

More info: https://sass-lang.com/d/color-functions

   ╷
37 │   @return red($value), green($value), blue($value);
   │                        ^^^^^^^^^^^^^
   ╵

We need to resolve these deprecations before frontend breaks.

Acceptance Criteria

• Fix Sass deprecations with no warnings after accessing pages.
• Deploy and verify frontend working as before.
• third

Additional context

• Breaking Change: @import and global built-in functions.
• Breaking Change: Color Functions.