feat: Replace deprecated import statements in Sass with new API

[dlnr]

Describe the pull request

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What

Applied the SASS recommended migrator to fix the @import deprecation.

Why

Warnings in build output.

How

npm install -g sass-migrator
sass-migrator module --migrate-deps index.scss

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[VincentSmedinga]

Thanks for the refactoring. I didn’t anticipate the namespace it would add; let’s decide on a good approach to keep the mixin names user-friendly.