Add !default to all variables

[maedi]

Feature Request

• I’ve read and understood the Contributing guidelines and have done my best effort to follow them.
• I’ve read and agree to the Code of Conduct.
• I’ve searched for any related issues and avoided creating a duplicate issue.
• I've searched the community forums to check for related issues or feature requests

Problem

On multiple occasions we have needed to override variables such as:

$AU-colordark-background-alt:       darken( $AU-colordark-background, 8% );

We can't easily do so because they have no !default flag. In our use case we need 4 unique variants, not just Standard and Dark, but Standard-Alt and Dark-Alt to be customisable variants.

Solution

It would be nice if every variable had a !default flag to avoid hacky methods of overriding them.