feat(DesignTokens): update overlap background color to color.neutral.900

mentioned in commit aa670cdd

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!51 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1067 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!349 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!167862 (closed)

mentioned in merge request gitlab!168179 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4098 (merged)

mentioned in merge request gitlab!168368 (closed)