fix(css): Allow overriding gray-a variables

mentioned in commit 09b02ec4

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!155 (merged)

mentioned in merge request status-page!817 (merged)

mentioned in merge request gitlab!124352 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3432 (merged)

mentioned in merge request gitlab-docs!3993 (merged)

mentioned in merge request gitlab-docs!3995 (merged)

mentioned in merge request status-page!820 (merged)

mentioned in merge request gitlab!124927 (merged)

mentioned in merge request gitlab-docs!4012 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3479 (merged)