Colocate Sass vars in their respective files

[mdo]

Moves most of the Sass variables into the component files. For forms and buttons, I'm going to create some centralized files most likely. Same for the overlays, like modal and offcanvas. I'll do that separately.

In case you're wondering why we'd do this, it makes customization via Sass very convenient.

// Example format for new variable default overrides
@use "theme" with (
  $text-color: white,
  $background-color: black,
);