fix(GlButton): Fix button text color in dark mode

mentioned in commit 1bbf63f9

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2838 (merged)

mentioned in merge request status-page!502 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2839 (merged)

mentioned in merge request status-page!503 (merged)

mentioned in merge request gitlab!86197 (merged)

mentioned in merge request gitlab!86222 (closed)

mentioned in merge request gitlab-docs!2619 (merged)