feat(ButtonGroup): Change border color separator in darkmode

mentioned in commit 4a566c31

mentioned in merge request gitlab!178190 (merged)

mentioned in merge request status-page!903

mentioned in commit bb2dbcac

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4259 (merged)

mentioned in merge request gitlab!180089 (closed)

mentioned in merge request gitlab!180099 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1485 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!64 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!367 (merged)