fix(BroadcastMessage): dark border in dark mode

mentioned in commit e59167d5

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!150 (merged)

mentioned in merge request status-page!796 (merged)

mentioned in merge request gitlab!121070 (merged)

mentioned in merge request status-page!797 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3407 (merged)

mentioned in merge request gitlab-docs!3885 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3408 (closed)

mentioned in commit x--/gitlab-ui@a91ea015

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!152 (merged)

mentioned in merge request gitlab!122801 (closed)