fix: use CSS custom property syntax

mentioned in commit 033aed52

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!40 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!645 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!338 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!159156 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3945 (merged)

mentioned in merge request gitlab!159431 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!726 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!783 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!811 (closed)