Some SCSS variable overrides no longer work in subtheme.

Problem/Motivation

Alterations to SCSS variables (that are not related to colour) in the files `variables.base.scss` or `variables.components.scss` within a subtheme, do not work since CivicTheme 1.11, without requiring the subtheme to override each component that uses those variables.

Steps to reproduce

1. Create a new subtheme, no new components.
2. Update the `$ct-border-radius` to `100%` in the subtheme's `components/variables.base.scss`
3. Compile, clear cache and review components in Drupal - the border radius will be using the civictheme's initial value, and not the new custom value.

Proposed resolution

Implement CSS variables to allow base theme components to use subtheme variable changes.