fix(css): chart legend display issue in dark mode

mentioned in commit 1b4bc141

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!231 (merged)

mentioned in merge request gitlab!136116 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!232 (merged)

mentioned in merge request gitlab!136198 (merged)

mentioned in merge request gitlab-docs!4385 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3604 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3605 (merged)

mentioned in merge request gitlab-docs!4387 (merged)

mentioned in merge request status-page!886 (merged)

mentioned in merge request status-page!888 (merged)