feat(GlModal): Use design tokens for colors

mentioned in commit 4129f7f1

mentioned in merge request status-page!903

mentioned in merge request gitlab!157555 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!338 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!40 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!645 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3931 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3932 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!726 (closed)