fix(charts): add background color to toolbox title

mentioned in commit a23ace94

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!104 (merged)

mentioned in merge request status-page!660 (merged)

mentioned in merge request gitlab-docs!3354 (merged)

mentioned in merge request gitlab-docs!3355 (closed)

mentioned in merge request status-page!661 (merged)

mentioned in merge request gitlab!106871 (merged)

mentioned in merge request gitlab-docs!3380 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3192 (merged)