Refactor CSS to use `@use` (mixins) instead of `@import`

[mikewuu]

          @mikewuu Should we follow this convention? https://sass-lang.com/guide

Partials
You can create partial Sass files that contain little snippets of CSS that you can include in other Sass files. This is a great way to modularize your CSS and help keep things easier to maintain. A partial is a Sass file named with a leading underscore. You might name it something like _partial.scss. The underscore lets Sass know that the file is only a partial file and that it should not be generated into a CSS file. Sass partials are used with the @use rule.

Originally posted by @razor-x in #47 (comment)