Change css rules

[hyanwong]

On building the site with grunt dev, I'm now getting some warnings, e.g.

Running "sass:dist" (sass) task
Deprecation Warning [legacy-js-api]: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0.

More info: https://sass-lang.com/d/legacy-js-api

Deprecation Warning [legacy-js-api]: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0.

More info: https://sass-lang.com/d/legacy-js-api

Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import

  ╷
1 │ @import "colour_scheme.scss";
  │         ^^^^^^^^^^^^^^^^^^^^
  ╵
    OZprivate/scss/content_index.scss 1:9  root stylesheet

Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import

  ╷
3 │ @import "ui_widgets.scss";
  │         ^^^^^^^^^^^^^^^^^
    OZprivate/scss/ui_widgets.scss 1:9  root stylesheet

Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use color.adjust instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
12 │ $grey-darker:            lighten($grey-base, 13.5%) !default; // #222
   │                          ^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    OZprivate/scss/_OZ_variables.scss 12:26  @import
    OZprivate/scss/OZ_specific.scss 1:9      root stylesheet

Deprecation Warning [color-functions]: lighten() is deprecated. Suggestions:

color.scale($color, $lightness: 13.5%)
color.adjust($color, $lightness: 13.5%)

More info: https://sass-lang.com/d/color-functions

These look like easy fixes for any new dev who wants to take a stab.