Common SCSS

In some cases, like in Pajamas Design System, we want to import common styles from GitLab UI, without reimporting Bootstrap's styles.

It's currently possible to import every required stylesheet:

@import '@gitlab/ui/scss/variables';
@import '@gitlab/ui/scss/mixins';
@import '@gitlab/ui/scss/components';

But this is error prone and involves additional maintenance costs if we were to add new common stylesheets that would need to be imported specifically as well.

By exposing a common.scss stylesheet, we're able to reduce code duplication in GitLab UI, and we make it easier to import GitLab UI's specific styles when Bootstrap is not required:

@import '@gitlab/ui/scss/common';