feat(GlButton): Update design tokens for darkmode

mentioned in commit 07a4b50d

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1194 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!169801 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4147 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1249 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)