Some things we can migrate, some others we need to duplicate and some others we don't need to do anything.
Variable definitions: duplicate them, so that current customizations relying on them keep working and can be easily migrated.
Custom styles: migrate them, the only thing relying on those styles are the components and they'd work just as well with a CSS native solution.
Mixins: leave them unchanged. There's no way to reproduce mixins in CSS (style queries could be a replacement, but it's not supported well yet – CSS native mixins are actively thought about, but it'll be a while until we get full support) If some of them are useful for custom styles, replace them with somewhat appropriate CSS variable equivalents.
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