Set CSS variables using configurable SASS variables

[folliehiyuki]

This PR introduces a better method to import the styling using the source SCSS file, instead of overwriting :root variables the CSS way. Note that the CSS output changes. This is because hsl() and hsla() are also SASS native functions, and they compile to the equivalent legacy CSS syntax here.

Example:

@use "<path_to_parvus.scss>" with (
  $background-color: #ffffff;
  $color: #000000;
);

Ref: https://sass-lang.com/documentation/at-rules/use/#configuration